A person pays Rs. 6.4 per liter of milk, He adds water and sells the mixture at Rs. 8 per liter, thereby making 37.5% profit. The proportion of water to milk received by the customer is (approximation)

Solution: Let the quantity of  milk purchased be x and quantity of  water added be y.  Then ratio of  water to milk y : x.

$\begin{array}{l}CP=6.4x\\ SP=8(x+y)\\ profit\%=37.5\\ Profit=(SP-CP)/CP\times 100\\ 37.5=\frac{8(x+y)-6.4x}{6.4x}\times 100\\ 8(x+y)=6.4x\times 1.375\\ \Rightarrow 8x+8y=8.8x\\ 8y=0.8x\\ \frac{x}{y}=\frac{10}{1}\\ y:x=1:10\end{array}$