SCREEN [2]

A shopkeeper gives old clothes at rate of ₹90 instead of ₹100
It can be said that on sale of ₹100 shopkeeper takes ₹10(i.e. ₹100 - ₹90) less
Dividing 100 in 100 parts we get 1 simliarly on dividing 10 in 100 parts we get 10/100
Thus for sale of ₹1 shopkeeper takes ₹10/100 less
Thus for sale of ₹100 clothes the shopkeeper takes (10/100) + (10/100) + ......(100 times)
Thus shopkeeper gives (10 / 100) * 100 = 10% discount if he sells clothes at ₹90 instead of ₹100
Thus it can be said that the shopkeeper gives 10% discount on sale of clothes of ₹100

SCREEN [3]

Similarly if the shopkeeper gives old clothes at rate of ₹23000 instead of ₹115000
It can be said that on sale of ₹115000 shopkeeper takes ₹92000(i.e. ₹115000 - ₹23000) less

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Dividing 115000 in 115000 parts we get 1 simliarly on dividing 92000 in 115000 parts we get 92000/115000
Thus for sale of ₹1 shopkeeper takes ₹ 92000/115000 less
Thus for sale of ₹115000 clothes the shopkeeper takes (92000/115000) + (92000/115000) + ......(115000 times)

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Thus shopkeeper gives (92000 / 115000) * 100 = 80% discount if he sells clothes at ₹23000 instead of ₹115000