Question

Leroi and Sylvia both put $100 in a savings account. Leroi decides he will put in an additional $10 each week. Sylvia decides to put in an additional 10% of the amount in the accounted each week

Answer 1

Sylvia puts in $121.

10% of (multiply) $210.
210= 100 each person plus an additional ten from Leroi
so .10*210 = $21. 
Add Sylvia's $100 plus $21 and viola, $121