On selling $17$ balls at $Rs. 720$,there is a loss equal to the cost price of $5$ balls. The cost price of a ball is (in $Rs.$)

Some articles were bought at $6$ articles for $Rs. 5$ and sold at $5$ articles for $Rs. 6$. The gain percent is: (in $\%$)

Sonal buys mangoes at the rate of $3 \, \text{kg}$ for $Rs. 21$ and sells them at $5 \, \text{kg}$ for $Rs. 50$. To earn a profit of $Rs. 102$, he must sell how many mangoes in $\text{kg}$?

Ramesh purchased a bicycle for $Rs. 5200$ and spent $Rs. 800$ on its repairs. He had to sell it for $Rs. 5500$. Find out his profit or loss percent.

When Ram sold his book for $Rs. 255$,he made a loss of $15\%$. For how much should he have sold it if he wanted to make a profit of $5\%$?

Seema sells a pen to Sapna at a profit of $25 \%$. Sapna sells it to Asha at a profit of $10 \%$ and Asha sells it to Kavita at a profit of $5 \%$. If Asha sells it for $Rs.\,231$,find the cost price at which Seema bought the pen? (in $Rs.$)