$A$ product when sold with $10 \%$ rebate on the listed price gave a profit of $Rs$ $70$. What was its cost price? (in $Rs$)

$A$ man bought $500$ metres of electronic wire at $50$ paise per metre. He sold $50 \%$ of it at a profit of $5 \%$. At what percent should he sell the remainder so as to gain $10 \%$ on the whole transaction?

An electric pump was sold at a profit of $15 \%$. Had it been sold for $Rs. 600$, the profit would have been $20 \%$. What was the former selling price in $Rs.$?

$A$ vendor started selling vegetables at $Rs. 10$ per $kg$,but could not find buyers at this rate. So,he reduced the price to $Rs. 8.10$ per $kg$,but uses a faulty weight of $900\, g$ for $1\, kg$. Find the percent change in the actual price or loss. (in $\%$)

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

$A$ vendor bought toffees at $6$ for a rupee. How many for a rupee must he sell to gain $20 \%$?