Question

By selling 80 mangoes a fruit seller gains the c. P. of 20 mangoes .find his gain%.

Answer 1

Here 
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Let The CP on 100 mangoes = SP of 80 mangoes 
Let the cost price on each mango = $1 
CP of 80 mangoes = $80 
SP of 80 mangoes  = $100
Gain  = SP - CP = $20
gain % = 1/4 x 100 = 25 %