UPSC CDS 2 2020 Math Paper

Given that a shopkeeper sells his articles at their cost price but by default balance which reads 1000 gm for 800 gm.
If he purchase and sell 1 kg article at 100 Rs. but due to faulty balance he gain 20 Rs at every 1 kg article because it only sell 800 gm. and its cost price for 800 gm is 80 Rs. and its sell price is 100 Rs for 800 gm because of faulty balance.
Then gain = 20
So, gain % =

gain×100

cost price

⇒ gain % =

20

80

×100
=25 %