At dorthys hair salon, 4 gallons of shampoo will last 3 weeks. she has 7 gallons of shampoo. how much more must she order if she wants to have enough for 12 weeks?

X=number of gallons that she must order.

4 gallons3 weeks
(7 gallons + x)12 weeks

We solve this rule of 3.
(7 gallons +x)3 weeks=4 gallons*12 weeks
21 gallons weeks+3 x weeks=48 gallons weeks
3x weeks=48 gallons weeks - 21 gallons weeks
3x weeks=27 gallons weeks
x=(27 gallons weeks) / 3 weeks=9 gallons.

Solution: she must order 9 gallons more.

A. is the answer in this question

These are the multiples of 15 but what you are probably asking or what are the factors of 15