Can somebody help me understand this one thoroughly Is an SA

Can somebody help me understand this one thoroughly. Is an SAT Prep question

There are g gallons of paint available to paint a house. After n gallons have been used. Then, in terms of g and n percent of the paint has not been used?

Solution

Initially we had g gallons of paint.

Now if n gallons are used then left amount = (g-n)

Therefore fraction of unused paint = left amount/ initial amount = (g-n)/g

If we change it to percentage then we need to multiply it by 100

Hence percentage of unused paint = (g-n)*100/g %

Therefore (E) is the correct option.