A rectangular pit 12 m long, 9 m wide and 4 m deep is dug for a foundation. Digging costs ₹35 per cubic metre, and the excavated earth is then sold for ₹12 per cubic metre. What is the net cost of the work?

The pit holds $12\times 9\times$ 4 = 432 cubic m. Digging costs $432\times 35$ = ₹15120 and the earth fetches $432\times 12$ = ₹5184, so the net outgo = 15120 - 5184 = ₹9936. Equivalently, $432\times (35-12)=432\times 23$ = ₹9936. The trap ₹15120 never credits the sale; ₹20304 adds the two amounts instead of subtracting; ₹5184 quotes only the money received.