The cost price of $100$ books is equal to the selling price of $60$ books. The gain or loss percentage will be:

Two discounts of $40 \%$ and $20 \%$ are equal to a single discount of what percentage?

$A$ trader allows a trade discount of $20 \%$ and a cash discount of $6 \frac{1}{4} \%$ on the marked price of the goods and gets a net gain of $20 \%$ of the cost. By how much above the cost should the goods be marked for the sale? (in $\%$)

$A$ person sold a radio at a loss of $5 \%$. Had he sold it for $Rs. 210$ more,he would have gained $25 \%$. For what value should he sell it in order to gain $35 \%?$

Aditya bought a scooter for a certain sum of money. He spent $15 \%$ of the cost price on repairs and sold it for a profit of $Rs. 1104$. What did he spend on repairs if the profit he earned was $20 \%$?

Amit purchased $13$ chairs for $Rs. 115$ each and sold all of them for $Rs. 1220$. Find the profit or loss on the transaction.