If the cost price of $12$ pens is equal to the selling price of $8$ pens,find the gain percent.

$A$ person sells $36$ oranges for $₹ 1$ and makes a loss of $4 \%$. Find how many oranges per $₹ 1$ should be sold to have a gain of $8 \%$.

Find $SP$ when $CP = Rs. 80.40$ and $\text{loss} = 5\%$.

$A$ shopkeeper purchases two items for $Rs. 520$. One of them is sold at a gain of $16\%$ and the other at a loss of $10\%$,thus making no profit or loss. What is the selling price of the item sold at a loss?

By selling an article for $Rs. 21$,a man incurs a loss such that the percentage loss is equal to the cost price. The cost price of the article is (in $Rs.$):

Surbhi bought a combined total of $25$ i-pads and i-phones. She marked up the i-pad by $20 \%$ on the cost price,while each i-phone was marked up by $Rs. 2000$. She was able to sell $75 \%$ of the i-pads and $2$ i-phones and make a profit of $Rs. 49000$. The remaining i-pads and $3$ i-phones could not be sold by her. Find her overall profit or loss if she gets no return on unsold items and it is known that an i-phone costs $50 \%$ of an i-pad.