Let selling price = SP and cost price = CP
He makes 20 % on selling price means = >
(sp-cp) / sp = 20 % = 20/100 = 0.2
(sp - cp) / sp = 0.2
1 - cp/sp = 0.2
1 - 0.2 = cp/sp
0.8 = cp/sp
Reversing 1 / 0.8 = sp/cp = 1.25
We need to find profit on cost price means =
=(sp-cp)/ cp
= sp/cp - 1 = 1.25 - 1 = 0.25
Means 25 % profit on cost price : Answer
Hope that will help :)