The marked price of a piano was $Rs. 15,000$. At the time of sale,there were successive discounts of $20\%$,$10\%$,and $10\%$ on it. The sale price was (in $Rs.$)

Ravi purchases $90$ pens and sells $40$ pens at a gain of $10 \%$ and $50$ pens at a gain of $20 \%$. Had he sold all of them at a uniform profit of $15 \%$,he would have got $Rs. 40$ less. Find the cost price of each pen in $Rs$.

$A$ manufacturer fixes his selling price at $33 \%$ over the cost of production. If the cost of production goes up by $12 \%$ and the manufacturer raises his selling price by $10 \%$,what is his new percentage profit?

$A$ trader marks the price of his goods $25 \%$ above the cost price but allows a $12.5 \%$ discount for cash payment. If he sells the goods for $Rs. 875$,find the cost price of the goods (in $Rs.$).

Ram purchased a computer set for $Rs. 12500$ and spent $Rs. 300$ on transportation and $Rs. 800$ on installation. At what price should he sell it to earn an overall profit of $15\%$? (in $Rs.$)

Successive discounts of $10 \%$,$20 \%$ and $50 \%$ will be equivalent to a single discount of .......... $\%$