Question:

If the selling price of 10 pens is the same as the cost price of 12 pens then gain per cent is

(a) 2%

(b) 12%

(c) 20%

(d) 25%

Solution:

(c) 20%

Let Rs $x$ be the SP of each pen.

SP of 10 pens $=$ CP of 12 pens $=$ Rs $12 x$

CP of 10 pens $=$ Rs $10 x$

Now, gain $=$ Rs $(12 \mathrm{x}-10 \mathrm{x})$

= Rs 12x

$\therefore$ Gain percentage $=\left(\frac{\text { gain }}{\mathrm{CP}} \times 100\right) \%$

$=\left(\frac{2 x}{10 x} \times 100\right) \%$

$=20 \%$