# Billy hearts some numbers

Billy likes the number 25 but not 35. He likes 144 but not 145. He likes 8100 but not 8000.

Does he like 872 or 841?

• I edited the question and replaced "25 but not 25" with "25 but not 24" since it looked like a typo. Feel free to roll it back in case I did it wrong. Jun 6 '16 at 13:57
• I actually put 25 but not 35. Jun 6 '16 at 18:11

841

Because

he likes perfect squares.
$841 = 29^2$

he likes 25 because

$25 = 5^2$

He likes 144 because

$144 = 12^2$

He likes 8100 becasue

$8100 = 90^2$

• This is only correct assuming the first line has a typo, and you probably shouldn't just edit someone's question like that Jun 6 '16 at 13:50
• @GordonAllocman it was a typo. I've seen this problem before. Here is one similar: free-iqtest.net/answers.asp?q=15 Jun 6 '16 at 13:51
• This is a very well known puzzle. Jun 6 '16 at 13:53
• Just because another similar question exists, doesn't mean it is exactly this one. While in all likelihood it was just a typo, you should really let OP fix it to be safe Jun 6 '16 at 13:54
• I agree. I guess I got an adrenaline rush because I knew the answer. I guess the OP can roll it back if there is a problem and earn a badge for "first rollback". Jun 6 '16 at 13:56

That was easy. Billy likes

841.

Clearly because,

Billy likes perfect squares. 841 is the square of 29.