A milkman bought 70 litres of milk for Rs. 630 and added 5 litres of water. If sells it at Rs. 9.00 per litres, his profit percentage is = ?

According to the question,
Cost price of 70 litres of milk = Rs. 630
Added 5 litres of water
Now, the solution becomes = 75 litres
Cost price of water = Rs. 0
∴ Selling price of 1 litre milk = Rs. 9
Selling price of 75 litres milk = 9 × 75 = Rs. 675
Profit = Selling price - Cost price
= 675 - 630
= 45
$$\eqalign{ & {\text{Profit}} = \frac{{\operatorname{Profit} }}{{{\text{Cost price}}}} \times 100 \cr & = \frac{{45}}{{630}} \times 100 \cr & = \frac{{50}}{7} \cr & = 7\frac{1}{7}\% \cr} $$