Question

a grocer sells 40 litre of oil for the cost price of 42 litre find his gain percent

Answer 1

Broad way :
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when cp is 40 gain is 2
when cp is 1 gain is 2/40
when cp is 100 gain is 2/40×100=5%
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NOW IN SHORT WAY :
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DEVIDE 100 IN THIS WAY 40+40+20
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SO THE PROFIT
40=2
40=2
20=1(as 20 is 1/2 of 40 the gain also should be half)
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so 2+2+1=5%

Answer 2

I thinks you could understand by this photo