Because you type incorrectly!

We only need two number: 17 and 16

(17 + 16 + 17) + (17 + 16 + 17)

We have:

99 = 72 + 27

45 = 27 + 18

39 = 18 + 21

36 = 21 + ??

28 = ?? + 13

21 = 13 + 7

(The number on the first row = The number below it + The number right next to the number below it)

So ?? is 15

72 cm²

Can I ask one question? Why this page is about Math, but the avatar is about chemistry?

Hey man, fb might be deleted in the future (I think so)!!

If you rearrange it, the sum of the digits is still 15, which makes it impossible to rearrange the digits of 3255 to make a number that is not a multiple of 3

The factorials that are divisible by 9 is 9!, 10!, 11!, 12!,..., 100! (These factorials contains factor 9)

=> There are: (100 - 9) + 1 = 92 (factorials)   

First we need to figure out if Albert knows the month or the day. If he knows the day, then there is no chance that Bernard knows the birthday, so it must be that Albert knows the month.

From the first statement, we know that Albert is sure that Bernard doesn't know the birthday, so May and June should be ruled out (the day 19 only appears in May and the day 18 only appears in June). In other words, if Albert had May or June, then he cannot be sure that Bernard doesn't know, since Bernard could have had 18 or 19.

Following that statement, Bernard knows that May and June are ruled out.

Then, Bernard is able to know which month it is. So it must be either July 16, August 15 and August 17 (not 14^th as then he can't know).

Since Albert subsequently can also be sure of the date, he must know it's July. If it's August, he can't be sure as there is August 15 and 17.

So the answer is July 16