A shopkeeper buys 80 kg of good quality mangoes and 40 kg of medium quality mangoes. He sells all the mangoes at a common selling price, which is 10% less than the price at which he bought the good on

Let the price of good quality mangoes be $P$ per kg.
The price of medium quality mangoes is half that of good mangoes, so it is $\frac{P}{2}$ per kg.

The shopkeeper buys 80 kg of good mangoes and 40 kg of medium quality mangoes.
Cost of good mangoes $= 80 \text{ kg} \times P \text{/kg} = 80P$.
Cost of medium quality mangoes $= 40 \text{ kg} \times \frac{P}{2} \text{/kg} = 20P$.
Total Cost Price (CP) $= 80P + 20P = 100P$.

The common selling price for all mangoes is 10% less than the price at which he bought the good ones ($P$).
Selling price per kg $= P - 10\% \text{ of } P = P - 0.10P = 0.90P$.

The total quantity of mangoes sold is $80 \text{ kg} + 40 \text{ kg} = 120 \text{ kg}$.
Total Selling Price (SP) $= 120 \text{ kg} \times 0.90P \text{/kg} = 108P$.

Profit $= \text{Total SP} - \text{Total CP}$
Profit $= 108P - 100P = 8P$.

Profit percentage $= \frac{\text{Profit}}{\text{Total CP}} \times 100\%$
Profit percentage $= \frac{8P}{100P} \times 100\%$
Profit percentage $= \frac{8}{100} \times 100\% = 8\%$.

The overall profit is 8%.

The final answer is $\boxed{\text{B) 8\%}}$.