White Christmas

Algebra Level 2

In Australia, the majority of rainfall happens in December to March when we are having our summer. In order to create a "White Christmas", we tear up a piece of paper into tiny pieces, and throw them up in the air.

Assume that we have the Incredible Hulk to help us. He can take any stack of paper, and tear the entire stack into 2, thereby doubling the number of pieces. Starting from a single piece of paper, what is the minimum number of tears that we must do, in order to get at least 1 , 000 , 000 , 000 1, 000, 000, 000 (really tiny) pieces of paper?


The answer is 30.

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10 solutions

Divye Baid
Dec 17, 2013

Since the Hulk is tearing down the entire stack into 2 2 , then 4 4 , then 8 8 , and so on,

2 x = 1 , 000 , 000 , 000 , 2^{x} = 1, 000, 000, 000, where x x is the number of times Hulk has to tear the stack into 2 2 .

2 x = 100 0 3 2^{x} = 1000^{3}

Since the power of 2 2 closest to 1000 1000 is 1024 1024 ,

2 x 2^{x} is about 102 4 3 1024^{3}

2 x 2^{x} is about ( 2 10 ) 3 (2^{10})^{3}

2 x 2^{x} is about 2 30 2^{30}

Therefore, the smallest whole number x x that gives 2 x > 1 , 000 , 000 , 000 2^{x} > 1, 000, 000, 000 is 30 30

okay.

Jay Cyril Mijares - 7 years, 5 months ago

i guess worked with my calculator till 30, and at last i got it right

Chitranshu Vashishth - 7 years, 4 months ago

log 1000000000/log2

Roelof Edelenbos - 7 years ago

Hey, min. total no. of tears is 1+2+3+...+30. Not just 30. Why are u not counting the previous tears ?

yhvh yhvh - 7 years, 5 months ago

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You don't count the sum of the tears. It's on the 30th tear that you get at least 1,000,000,000 pieces.

Klahrinz William Catubig - 7 years, 5 months ago
Vishal Jindal
Dec 17, 2013

as we keep dividing by 2 it forms a progression 2^n > 10^9

taking log on both sides n * log2 = 9

n = 9/log 2

the corresponding integer value is 30

Sorry, I was wrong!It actually is not.The table was incorrect...

Bogdan Simeonov - 7 years, 5 months ago

check first before you type

Jay Cyril Mijares - 7 years, 5 months ago

But 2 on the power of 29 is bigger than 1,000,000,000 , so the answer is not true!I checked the powers of 2 table

Bogdan Simeonov - 7 years, 5 months ago

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Hey! Who said 2^29 .1000000000. 2^29= 536870912.

Pankaj Joshi - 7 years, 5 months ago

For x x tears, the Incredible Hulk can produce 2 x 2^x pieces of paper.

The problem wants us to find the smallest natural value of x x such that the inequality 2 x 1 0 9 2^x \geq 10^9 holds.

Applying log 2 \log_2 to both sides, we get that x 9 + 9 × log 2 5 x \geq 9 + 9 \times \log_25 . If I know that log 10 2 0.3 \log_{10}2 \approx 0.3 , we can say that log 2 5 l o g 10 5 l o g 10 2 0.7 0.3 = 2. 3 \log_25 \Rightarrow \frac{log_{10}5}{log_{10}2} \approx \frac{0.7}{0.3} = 2. \overline{3} . Thus, x 29. 9 x = 30. x \geq 29. \overline{9} \Rightarrow \boxed{x = 30.}

same method :)

Happy Melodies - 7 years, 5 months ago
Milind Prabhu
Mar 17, 2014

Every time the paper is divided into 2.

so the equation is 2 x = 1000 3 { \quad 2 }^{ x }={ 1000 }^{ 3 }

32 2 = 1024 { 32 }^{ 2 }=1024

32 happens to be a power of 2

2 10 = 1024 { 2 }^{ 10 }=1024

2 30 { 2 }^{ 30 }\quad is obviously about 1 , 000 , 000 , 000 1,000,000,000

Therefore the paper must be halved 30 times.

Khaled Mohamed
Feb 12, 2014

the sequence we have is : (1.2.4.8......) "Geometric sequence"

then: a=1

r= 2/1 = 4/2 = 8/4 .....=2

Sum = 1,000,000,000

sum = a(1-r^n / 1-r)

1,000,000,000 = 1x ( 1-2^n / -1) ------> by solving, n = 29.897 = 30

Finn Hulse
Jan 23, 2014

Our equation is two to the x power is greater than 1000000000. First we will solve for what value of x actually gets 1000000000. Let's use our logarithmic properties: x will be equal to log(1000000000) divided by log(2). Well, log(1000000000) just equals 9, so divided by log(2) equals 29.897. The next integer above that is 30.

Deep Agarwal
Jan 19, 2014

1 piece of paper when torn will give u 2 pieces..... hence 1=2^1 2=2^2 3=2^3 ... ... ... ... n=2^n ...given here is total pieces obtained.and we have to find torn we made hence
2^n=1,000,000,000 taking log on both sides n* log2=log(1,000,000,000) .... n * 0.3=9 ..... n=9/0.3 n=30

On the first tear, it yields two pieces, on the second it yields 4, one the third it yields 8, and so on. So 2^{n} = (number of pieces)

2^{34} = 1x10^{9}

log {2}(2^{n}) = log {2}1x10^{9}

n = log_{2}1x10^{9}

n ≈ 30

Abishanka Saha
Dec 20, 2013

We have to find k k such that 2 k > 1 , 000 , 000 , 000 2^k>1,000,000,000 but 2 k 1 < 1 , 000 , 000 , 000 2^{k-1}<1,000,000,000 . So k = 30 k=30 as log 2 ( 1 , 000 , 000 , 000 ) 29 \log_2(1,000,000,000)\sim29 .

Santanu Banerjee
Dec 17, 2013

Since 2^29<1000000000<2^30

Answer is 30

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