$A$ trader purchases a wrist watch and a pendulum for $Rs. 390$. He sells them making a profit of $10 \%$ on the watch and $15 \%$ on the pendulum. He earns a total profit of $Rs. 51.50$. The difference between the original prices of the pendulum and the watch is equal to? (in $Rs.$)

$A$ man purchased $150$ pens at the rate of $Rs. 12$ per pen. He sold $50$ pens at a gain of $10 \%$. The percentage gain at which he must sell the remaining pens so as to gain $15 \%$ on the whole outlay is........$\%$

On selling an article for $Rs. 500$,the loss incurred is $20 \%$. To make a profit of $20 \%$,the article must be sold at what price in $Rs.$?

$A$ cloth dealer professes to sell cotton at cost price, but uses a meter scale having a length of $80 \, cm$ only and charges for a full meter. Find his gain percent.

$A$ shopkeeper purchased $150$ identical pieces of furniture at the rate of $Rs. 250$ each. He spent an amount of $Rs. 2500$ on packing. He fixed the price of each furniture at $Rs. 320$. However,he decided to give a discount of $5 \%$ on the labeled price. What is the percent profit earned by him (in $\%$)?

$A$ shopkeeper marks his goods at $40 \%$ above their cost price. He is able to sell $\frac{3}{4}$ of his goods at this price,and the remaining at $40 \%$ discount. Assuming that the shopkeeper is able to sell all the goods he buys,find his overall gain or loss percentage.