A dishonest dealer professes to sell his goods at Cost Price but still gets 20% profit by using a false weight. What weight does he substitute for a kilogram ?

Let the error is x gm. So we have
( error / True weight – error ) * 100 = %gain profit
= ( x / 1000 – x ) * 100 = 20
x / 1000 – x = 1 / 5
5x = 1000 – x
6x = 1000
x = 1000/6 gm

Hence the weight = 1000 – (1000 / 6)
833 1/3