Question

A vendor sold 1/3 of his fish balls in the afternoon and 2/5 of the remainder in the evening. If he had 150 fish balls left, find the number of fish balls he had at first?

Answer 1

let the number of fish at the start be x
sold (1/3)x in the afternoon, leaving his with (2/3)x
He sold 2/5 of that leaving him with
(3/5) (2/3)x or (2/5)x

(2/5)x = 150
x = 375

check:
sell 1/3 of 375 = 125 , leaves 250
sells 2/5 of that or 100 , leaving him with 150
All is good!

Answer 2

n - n/3 - 2/5 (2n/3) = 150
n - n/3 - 4n/15 = 150
(15 - 5 - 4)n/15 = 150
6n/15 = 150
2n/5 = 150
n = 375

check:
sold 1/3 of 375 = 125, leaving 250
sold 2/5 of 250 = 100, leaving 150