Denim Company offers sales people the following commission structure, with commission being paid monthly:

1.00% on the first $2,000 they sell, 4.00% on the next $3,000 they sell, and
4.50% thereafter on all sales they make.

Nathan, a new sales representative, made sales of $2,500 in her first month, $6,000 in her second month, and $5,200 in her third month.

a) what were her total commissions for the first three months?

b) what single commission rate would represent her total earnings throughout the first three months?