A man buys 21 mangoes for a rupee. How many mangoes had he sold for a rupee so that there is loss of 30%?

30

CP of one mango = Rs 1/21

We use ratios to solve this question,

To incur a loss of 30%,

if the base is 100, SP = 100 - 30 = 70

The ratio between CP and SP will be

                       CP   :    SP

                      100  :    70

                        ↓

                       1/21

Then, SP = 70 x $\frac{1}{100 \times 21}$ = $\frac{1}{30}$

Thus, he needs to sell 30 mangoes for a rupee to incur a loss of of 30%