Explanation:
x = original number of sales
income = x * p now increase the price to 1/3 more
income = x * 1 1/3 p now DECREASE the number of sales
income = 3/5 x * 1 1/3 p and simplify
3/5 * 4/3 * x * p
12/15 x * p
4/5 x * p
this 4/5 of the original x * p
a DECREASE of 1/5