Question

If 10 pens cost 60 rupees, what is the cost of 13 such pens?

Answer 1

Let us suppose the cost of 13 pens is x rupees.
The number of pens and their cost vary
in direct proportion. Let us express the
ratios and obtain an equation.

$\begin{array}{cc}\frac{10}{60}& =\frac{13}{x}\\ \therefore 10x& =780\text{(multiplying both sides by}\\ \therefore x& =7860x)\\ & \text{Cost of}13\text{pens is}₹78\end{array}$