Commercial Mathematics

On selling an article for $$Rs-2,640$$, profit of $$10$$ percent is made:
Find (i) cost price of the article.
Why do we need an index number?
Find the value of $$9\%$$ of $$8400$$ kg.
A man invests Rs. $$46875$$ at $$4\%$$ per annum compound interest for $$3$$ years.
A man invests Rs. $$46875$$ at $$4\%$$ per annum compound interest for $$3$$ years.