Problem Solving

A paint crew gets a rush order to paint 80 houses in a new development. they paint the first y houses at a rate of x houses per week. Realizing that they'll be late at this rate, they bring in some more painters and paint the rest of the houses at the rate of 1.25x houses per week. The total time is takes them to paint all the houses under this scenario is what fraction of the time it would have taken if they had painted all the houses at their original rate of x houses per week?

(A) 0.8(80-y)
(B) 0.8+0.0025y
(C) 80/y-1.25
(D) 80/1.25y
(E) 80-0.25y

The OA is B.

Please, can any expert explain this PS question for me? I have many difficulties to understand why that is the correct answer. Thanks.