Let us assume the car which is 40% more popular than black is a and total number of car is P.
According to question,
(P x a/100 ) - (P x 5/100) = P x 5/100 x 40/100
P(a - 5)x1/100 = P x 5/100 x 40/100
(a - 5) = 5 x 40/100
(a - 5) = 200/100 = 2
a - 5 = 2
a = 2 + 5 = 7
40% more popular than Black cars = 7% popular Silver cars.
