A problem by Zi Song Yeoh

Level pending

An integer n n is written on a board. Then, in every minute, if n = a + b n = a + b , then n n is replaced by a b ab .

Determine the number of positive integers n 1000 n \le 1000 such that the number 2013 2013 can be attained.


The answer is 996.

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.

2 solutions

Sadman Sakib
Dec 14, 2013

2013 = 3 × 11 × 61 = 33 × 61 2013 = 3 \times 11 \times 61 = 33 \times 61

So, if we can attain at least 94 94 then we are done on the next minute . Confused with at least ? Just note that from n + 1 n + 1 we can easily attain n n . Thus, we have to rule out positive integers from which we can't go greater than 93 93 .

Consider n 4 n \leq 4 . For this case we can't get integers greater than 4 4 . And for every n 5 n \geq 5 we can hop and jump to reach integers greater than 93 93 and then 94 94 by the method . It concludes that we can attain 2013 2013 if 5 n 1000 5 \leq n \leq 1000 .So, the answer is 1000 4 = 996 1000 - 4 = \boxed{996}

Daniel Chiu
Dec 14, 2013

Note that 2013 = 3 11 61 = 33 61 2013=3\cdot 11\cdot 61=33\cdot 61 , so 33 + 61 = 94 33+61=94 can make 2013.

Also, if n > 4 n>4 , then 2 ( n 2 ) = 2 n 4 > n 2(n-2)=2n-4>n is achievable, so n n can be increased arbitrarily.

In addition, 1 ( n 1 ) = n 1 < n 1(n-1)=n-1<n is achievable, so n n can be decreased arbitrarily.

Then, any 4 < n 1000 4<n\le 1000 works. Since 1,2,3,4 cannot be increased past themselves, the answer is 996 \boxed{996} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...