Did you learn how to subtract?

Level pending

The decimal parts of two irrational numbers in ( 0 , 1 ) (0,1) taken at random are found to 2014 2014 places. The probability that the smaller one can be subtracted from the other without borrowing can be expressed as p k p^k , where p p is a rational number and k > 1000 k > 1000 is a positive integer. Find the last three digits of 100 ( p + k + 1 ) \lfloor 100(p+k+1) \rfloor .

Assume for this question that the digit in first decimal place of the larger number is greater than the digit in that of the other. (As a follow up, can you say what would the probability be, without this assumption?)


The answer is 455.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Let the two randomly chosen irrational numbers be α = 0. a 1 a 2 a 3 a 2014 \alpha = \overline{0.a_1a_2a_3 \cdots a_{2014}} and β = 0. b 1 b 2 b 3 b 2014 \beta = \overline{0.b_1b_2b_3 \cdots b_{2014}} where a i , b i { 0 , 1 , 2 , , 9 } a_i,b_i \in \{0,1,2, \cdots ,9\} for 1 i 2014 1 \leq i \leq 2014 . Suppose α > β \alpha > \beta . For subtraction of β \beta from α \alpha without borrowing, a i b i a_i \geq b_i .

Therefore, for 1 i 2014 1 \leq i \leq 2014 , a i { b i , b i + 1 , , 9 } a_i \in \{b_i,b_i+1, \cdots ,9\} . Thus after choosing b i b_i , a i a_i can be chosen in ( 10 b i ) (10 - b_i) ways. So the number of favourable cases, given any b i b_i , is j = 0 9 ( 10 j ) = j = 1 10 j = 55 \sum_{j=0}^{9} (10 - j) = \sum_{j=1}^{10} j = 55 . The total number of ways of selection of a i a_i and b i b_i is 10 × 10 = 100 10 \times 10 = 100 .

Thus the required probability is (for 2014 2014 digits) = ( 55 100 ) 2014 = (\frac{55}{100})^{2014} . Thus 100 ( p + k ) = 100 ( 0.55 + 2014 ) = 201455 100(p+k) = 100(0.55 + 2014) = \boxed{201455} . \blacksquare

But the question asks for 100 ( p + k + 1 ) \lfloor 100(p+k\boxed{+1})\rfloor ...

EDIT: Okay I see below

Daniel Chiu - 7 years, 4 months ago

(Even with your edit of k = 2013 k=2013 )

Why must be have p = . 55 p=.55 and k = 2013 k= 2013 ?

Why can't we have p = . 5 5 3 p = .55^3 and k = 2013 3 k = \frac{2013}{3} ?

Calvin Lin Staff - 7 years, 4 months ago

Log in to reply

That is a problem I often face while mending a question for accepting numerical answers, thanks Sir. Let's say k > 1000 k>1000 .

A Brilliant Member - 7 years, 4 months ago

Since α β \alpha \geq \beta , wouldn't we have a 1 b 1 a_1 \geq b_1 by default?

Siddhartha Srivastava - 7 years, 4 months ago

Log in to reply

Yes, you are correct. We cannot have 100 100 choices for { a 1 , b 1 } \{a_1,b_1\} . There are 55 55 valid and possible choices for this pair. So k = 2013 k = 2013 basically. The question has been changed. Thanks for noticing,

A Brilliant Member - 7 years, 4 months ago

Log in to reply

I hate to be a pain but, if a 1 = b 1 a_1 = b_1 , Then we have a 2 b 2 a_2 \geq b_2 . And as long as a j b j a_j \leq b_j for all 1 j < i 1 \geq j < i , We have a i b i a_i \geq b_i for i 2014 i \leq 2014

And your original question would have been correct ( the ( 55 100 ) 2014 (\frac{55}{100})^{2014} one ) if you replaced the clause "the smaller one" with "the first one".Sorry for the inconvenience (which I'm sure must be a lot).

Siddhartha Srivastava - 7 years, 4 months ago

Log in to reply

@Siddhartha Srivastava When I thought of this question, it was to subtract one of them from the other. However, since subtracting a larger number from a smaller one, has complex relations with respect to taking borrow & so, so I changed the question a little. But I see the same problem is arising. I am adding a detail.

A Brilliant Member - 7 years, 4 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...