If the selling price of 10 pens is equal to cost price of 14 pens,

Let the cost price of one pen be Rs. C, and the selling price be Rs. S

Therefore, $10 \mathrm{~S}=14 \mathrm{C}$

$\mathrm{C}=\frac{10}{14} \mathrm{~S}$

However, the cost price is less than the selling price.

S. P. $=\left(\frac{100+\text { profit } \%}{100}\right)$ C.P

S $=\left(\frac{100+\text { profit } \%}{100}\right)$ C

$\frac{\text { S }}{\text { C }}=\left(\frac{100+\text { profit } \%}{100}\right)$

$\frac{14}{10}=\left(\frac{100+\text { profit } \%}{100}\right)$

$\frac{1400}{10}=100+$ profit $\%$

$140-100=$ profit $\%$

Profit $\%=40$

$=40 \%$

Therefore, the required profit percent is $40 \%$.