A vendor threw away 12% of the mangoes he had bought because they were unfit, and sold 80% of the rest. If 176 mangoes were left unsold, how many had he bought?

Let him have bought $M$ mangoes. After throwing away 12%, 0.88$M$ remain, and the unsold fifth of these is $0.20\times 0.88M=0.176M=176$, so $M=176/0.176=1,000$. Check: 1000 -> 880 fit -> 176 left unsold. 880 is 176/0.20, which ignores the 12% thrown away. 2,200 adds 12% and 80% to 92% and leaves 8%, but the 80% applies only to the survivors. 200 divides by 0.88 alone.