Question

Alex starts skip counting at 13 and writes 100 numbers. The 25th number he writes after writing 13 is 88. How many odd numbers does Alex write?

Answer 1

50

Explanation:

We are creating an algebraic series, i.e. picking numbers that are are constant steps away from each other.

If at 25 steps, we have jumped `88 - 13 = 75` steps, each step is clearly 3 steps. Therefore, every number switches between even and odd. Therefore, the first number is odd and the third too and all the way to the ninety-ninth.

That means that 50 of the numbers written down are odd and 50 are even.