The pattern is as follows:
2, 6, 12, 20.
For each tower, we can see we have the X2 + X - where X is the tower number.
So, let's say it was the 3rd tower.
So, substituting for the equation, we get:
32 + 3 = 12
If we check, that is true. The 3rd tower does have 12 blocks.
Let's find the total for the 99th tower.
Total = 992 + 99 = 9900
Therefore, the 99th tower has 9900 blocks.
(Solved by a 12 y/o)