now, we're assuming she made the deposit at the child's birth, namely when year was 0, at the child's twenty-first birthday that'll be 21 years later.

Here's how to figure out this problem: add up all the numbers in the mark category and divide by the # of numbers there are.
So....
6 + 7 + 8 + 9 + 10 ÷ 5 = 8
8 is your final answer.