A man sells 16 mangoes at a cost price of 20 mangoes, then percentage of his gain is?

Let the cost price of 20 mangoes = Rs. N
CP of 1 mango = Rs. N/20
SP of 1 mango = Rs. N/16
Gain = SP - CP = N/16 - N/20 = Rs. N/80
Gain percentage = {(N/80) / (N/20)} x 100 % = 25%