A grocer wants to mix tea costing Rs.40 per kg with tea costing Rs.45 per kg so that the mixture costs Rs.41.50 per kg. What should be the ratio of cheaper tea to dearer tea?

Using alligation, $\frac{\text{Quantity of cheaper}}{\text{Quantity of dearer}}=\frac{45-41.5}{41.5-40}=\frac{3.5}{1.5}=\frac{7}{3}$. So the cheaper and dearer teas must be mixed in the ratio $7:3$.