A retailer increase the selling price by 25% due to which his profit percentage increases from 20% to 25%. What is the percentage increase in cost price?

In beginning
Cost Price (CP) = Rs. 100
Profit % = 20
Selling Price (SP) = 120
When profit increases from 20% to 25%
Cost Price (CP) = y
Profit % = 25
Selling Price (SP) after 25% increase = 120 + 25% = Rs. 150
From question
y + 25 % profit = 150
1.25 y = 150
so y = 120
% change in cost price = [ (120 - 100) / 100 ] x 100 = 20%