A child is making a 5-level pyramid by stacking some blocks. In the first row, he has 9 blocks. For the next rows, he has 2 less than the blocks he has from its bottom row. How many blocks did he use in all?
Thoughts? Step-by-step reasoning:
On the second row, he used 9 - 2 = 7 blocks. On the third row, he used 7 - 2 = 5 blocks. On the fourth row, he used 5 - 2 = 3 blocks. On the fifth row, he used 3 - 2 = 1 block. Therefore, he used a total of 9 + 7 + 5 + 3 + 1 = 25 blocks.
Thus, the answer is 25.