Repeating New Year

0.20 16 = 0.2016161616 = a b 0.20\overline{16}=0.2016161616\ldots =\frac{a}{b}

Given that a a and b b are coprime positive integers, find a + b a+b .


Inspiration .


The answer is 2974.

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6 solutions

Henny Lim
Dec 30, 2015

I consider that 0.20161616... is made up from 0.04 plus 0.16161616...

0.04 is 1/25

0.161616... is 16/99

Sum both numbers up and I got 499/2475.

Then sum both nominator and denominator, I got 2974.

Slightly different approach but I think this solution is much similar with Lee's.

Very elegant. Very simple. Nicely done.

Tom Lawton - 3 years, 3 months ago

How do you know that 0.161616... is 16/99 ? What kind of math is that, and is there a good reference book on it?

bp kline - 2 years, 7 months ago

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With this kind of repeating decimal, there is a simple pattern that can be followed to convert it to a fraction. Here are a few examples:

2/9 = 0.222222... 5/9 = 0.555555...

But here comes the interesting part:

34/99 = 0.343434... 238/999 = 0.238238... 2048/9999 = 0.20482048...

Essentially, the number in the numerator repeats to the right of the decimal point. Notice that the number of 9's has to be the same as the number of digits in the numerator, otherwise you'll get a leading zero in the repeating decimal:

24/999 = 0.024024...

But having the numerator greater than or equal to the denominator does not work:

24/9 does not equal 2.424242.

Jeremy Cutler - 2 years, 5 months ago

It's a geometric series: 0.161616 = i = 1 + 16 1 10 0 i = 16 1 100 1 0.161616\ldots = \sum_{i=1}^{+\infty} 16 \frac{1}{100^{i}} = 16\frac{1}{100-1}

Miguel B - 2 years, 4 months ago

let b = 0.1616... ; then 100b = 16.1616... (100, because that draws one occurrence of the repeating pattern in front of the decimal) Subtract. All decimals cancel out: 99b = 16

Bas Cost Budde - 2 years, 4 months ago
Chan Lye Lee
Dec 28, 2015

Let x = 0.2016161616 x=0.2016161616\ldots .

Then 9900 x = 10000 x 100 x = 2016.16161616 20.1616161616 = 2016 20 9900x=10000x-100x=2016.16161616\ldots - 20.1616161616\ldots=2016-20 .

This means that x = 2016 20 9900 = 499 2475 = a b x=\frac{2016-20}{9900}=\frac{499}{2475}=\frac{a}{b} and hence a + b = 499 + 2475 = 2974 a+b=499+2475=2974 .

How did you make the observation that 9900x would produce a whole number? Just curious.

Skye Chen - 5 years, 5 months ago

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see he or atleast me noticed that in order to recieve a whole no we need to cancel out the decimal repeating part of the number so its better to convert into two distinct forms in such a way that subtracting them yields a whole number so we did this.btw,it follows from a general form to convert any repeating into fractional form

Kaustubh Miglani - 5 years, 5 months ago

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Thank you!

Skye Chen - 5 years, 5 months ago

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@Skye Chen u can follow me for more such comments and good questions

Kaustubh Miglani - 5 years, 5 months ago

x = 0.20 16 = 0.20161616 { 1 0 4 x = 2016.161616 = 2016. 16 ( 1 ) 1 0 2 x = 20.161616 = 20. 16 ( 2 ) x=0.20\overline{16}=0.20161616\ldots\implies\begin{cases}10^4x=2016.161616\ldots=2016.\overline{16}\qquad(1)\\ 10^2x=20.161616\ldots=20.\overline{16}\qquad\qquad(2)\end{cases}

What do you get when you subtract equation ( 2 ) (2) from equation ( 1 ) (1) ?

The motivation to do this is usually noticing the repeating pattern and see how we can cancel out the repeating parts to get a linear equation in one variable (the variable representing the original number) that has a simple rational solution for x x , which would be the rational representation of that number.

Prasun Biswas - 5 years, 5 months ago

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Thanks, I actually think this is a better answer. Asdfgh Rfcv, your answer was good too but it didn't really explain. Thank you both for helping!

Skye Chen - 5 years, 5 months ago

the periodic part are gone after the subtraction.

Denis Schüle - 3 years ago

Same solution!

A Former Brilliant Member - 5 years, 5 months ago

I still blur

naveen naveen - 4 years, 7 months ago

I found 121 600 \frac{121}{600} , that both seems to be co-primes. I was so disappointed that 121 600 \frac{121}{600} not being the right answer that I throw away part of my calculations, but I have used these formulas of 'Generating all CoPrime Pairs' at wikipedia: https://en.wikipedia.org/wiki/Coprime_integers

Is it the (121, 600) pair also a possible one?

Victor Delgado - 4 years, 3 months ago

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Ops I got my error now, 0.2016161616..., not 0.2016666666... that gives my wrong answer. So, nevermind my former question. Nice solution above.

Victor Delgado - 4 years, 3 months ago

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I did the same mistake, was confused for a very long time.

Martin Brause - 3 years, 3 months ago

I totally forgot about the coprime part :-D

Denis Schüle - 3 years ago

It's easier to see if you start with 100x= 20.16161616...

You multiply with the power of 10, which shifts the decimal expansion by the length of the recurring portion. Because then subtracting the original number x=0.2016161616... you get rid of these repeating decimals and have 99x=19.96. Now it becomes obvious you need 9900x to get an integer number 1996 and 1996/9900 is the decimal expansion we started with, which can be reduced to 499/2475.

Olaf Doschke - 2 years, 10 months ago
Jack Rawlin
Dec 31, 2015

First we split the decimal into two parts, the repeating part and the non-repeating part.

0.20 16 = 0.2 + 0.00 16 0.20\overline{16} = 0.2 + 0.00\overline{16}

We then turn both of them into fractions.

0.2 + 0.00 16 = 20 100 + 16 99 100 = 20 100 + 16 9900 0.2 + 0.00\overline{16} = \frac{20}{100} + \frac{\frac{16}{99}}{100} = \frac{20}{100} + \frac{16}{9900}

Then we simplify

20 100 + 16 9900 = 99 20 + 16 9900 = 1996 9900 \frac{20}{100} + \frac{16}{9900} = \frac{99\cdot20 + 16}{9900} = \frac{1996}{9900}

1996 9900 = 998 4950 = 499 2475 \frac{1996}{9900} = \frac{998}{4950} = \frac{499}{2475}

Then we solve

499 2475 = a b \frac{499}{2475} = \frac{a}{b}

a = 499 , b = 2475 a = 499, b = 2475

Therefore

a + b = 2974 \boxed{a + b = 2974}

nothing in the given stated how many digits we had to take into consideration.

Sacha Guitry - 3 years, 4 months ago

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It's a recurring decimal, you can take all of the digits into consideration, it's only when the decimal is either terminating (limited number of decimal places) or irrational (no pattern to an infinite number of decimal places) that you need to take into account the number of digits you need to be working with.

For recurring decimals they can be represented by sums, for instance 0.1 6 0.1\overline{6} (otherwise known as 1 6 \frac{1}{6} ) can be written as:

0.1 6 = 0.1 + 0.06 + 0.006 + 0.0006 + 0.1\overline{6} = 0.1 + 0.06 + 0.006 + 0.0006 + \cdots

Ignoring the 0.1 0.1 it's easy to see the pattern the sum is taking and from that we can find the fraction that sum represents.

0.06 + 0.006 + 0.0006 + = 6 100 + 6 1000 + 6 10000 + 0.06 + 0.006 + 0.0006 + \cdots = \frac{6}{100} + \frac{6}{1000} + \frac{6}{10000} + \cdots

First we multiply by 10 10 , we'll call the original sum x x for now

10 x = 6 10 + 6 100 + 6 1000 + 6 10000 + 10x = \frac{6}{10} + \frac{6}{100} + \frac{6}{1000} + \frac{6}{10000} + \cdots

We know the righthand side of the equation contains x x due to the simple fact that x x is an infinite sum.

10 x = 6 10 + x 10x = \frac{6}{10} + x

From here we subtract x x and divide by 9 9

9 x = 6 10 9x = \frac{6}{10}

x = 6 90 x = \frac{6}{90}

From here we just simplify

x = 3 45 x = \frac{3}{45}

x = 1 15 x = \frac{1}{15}

This isn't 1 6 \frac{1}{6} of course but that's only because we haven't added the 0.1 0.1 back in from when we took it out earlier

0.1 6 = 0.1 + 1 15 0.1\overline{6} = 0.1 + \frac{1}{15}

0.1 6 = 1 10 + 1 15 0.1\overline{6} = \frac{1}{10} + \frac{1}{15}

0.1 6 = 15 150 + 10 150 0.1\overline{6} = \frac{15}{150} + \frac{10}{150}

0.1 6 = 25 150 0.1\overline{6} = \frac{25}{150}

Finally we simplify

0.1 6 = 5 30 0.1\overline{6} = \frac{5}{30}

0.1 6 = 1 6 0.1\overline{6} = \frac{1}{6}

And there we have it.

If you get stuck on questions like these in the future try and remember this method as it works for every recurring decimal you might come across.

If you need anything else explaining just respond below this comment and I'll try to respond back, I'm not as active on here as I used to be so it might be a while before you get another response.

Hope I helped!

Jack Rawlin - 3 years, 4 months ago

Well, yes. The bar notation does. Infinite repetition.

Bas Cost Budde - 2 years, 4 months ago

224/1111 224+1111=1335

Anton Amirkhanov - 1 year, 9 months ago
Abe Morillo
Dec 29, 2015

I used same solution.

ChaoticMC Angel
Apr 13, 2018

0.20161616...

= 0.2 + 0.01616...

= 1/5 + 16/9900

= 1980/9900 + 16/9900

= 1996/9900

= 499/2475

Now we have:

499 + 2475

= 2974

Chen Yu
Nov 25, 2016

These are really too easy for someone who has done a maths undergradute, not to mention people with doctoarte in mathematics (statistics).

Don't be rude.

Greg S - 4 years, 4 months ago

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Sorry. Was not trying to be rude. But this is really too simple. We do those as extension questions in primary schools (Shanghai/China).

Chen Yu - 4 years, 4 months ago

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