Question

If the cost of 15 pens of the same value is Rs 600, find the cost of -
(i) 20 pens (ii) 3 pens.

Answer 1

Let us denote the required cost by x. Now, writing the like terms together, we have :
$\left(i\right)\begin{array}{cc}\text{No. of Pens}& \text{Cost in rupees}\\ 15& 600\\ 20& x\end{array}$
$\text{Ratio of pens}=\frac{15}{20}=\frac{3}{4}$
$\text{Ratio of rupees}=\frac{600}{x}$
Since, more pens cost more money, so this is a case of direct variation
Therefore , $\frac{3}{4}=\frac{600}{x}$
$\Rightarrow 3\times x=600\times 4$
$\Rightarrow x=\frac{600\times 4}{3}=200\times 4=800$
$\therefore$ The cost of 20 pens is Rs 800.
$\text{(ii) Again, ratio of pens}=\frac{15}{3}=\frac{5}{1}$
$\text{ratio of rupees}=\frac{600}{x}$
$\therefore \frac{5}{1}=\frac{600}{x}$
$\Rightarrow 5\times x=600\times 1\Rightarrow x=\frac{600}{5}=120$
$\therefore$ The cost of 3 pens is Rs 120.