A trader sells three-fourths of his stock of jackets at a profit of 20% and the remaining one-fourth at a loss of 10%. On the whole he gains ₹3600. His total cost price (in ₹) was:

Let the total cost be $C$. Three-fourths of it returns $0.75C\times 1.20=0.90C$ and the last fourth returns $0.25C\times 0.90=0.225C$, a total of $1.125C$. The gain is therefore $0.125C=3600$ → $C$ = ₹28800. (Averaging +20% and −10% into a 5% overall gain gives the trap ₹72000 and ignores that the profitable part is three times as large as the loss-making part.)