If $\rm \frac{21\ cosA+3\ sinA}{3\ cosA+4\ sinA}$ = 2, then find the value of cot A

A can complete a piece of work alone in 120 days, while A and B, working together, can complete this piece of work in 72 days. In how many days can B, working alone, complete this piece of work?

The interest earned on Rs. 21,000 in 3 years at simple interest is Rs. 6,400. What is the rate of interest per annum?

Each inlet pipe can fill an empty cistern in 84 hours while each drain pipe can empty the same cistern from a filled condition in 105 hours. When the cistern is empty, 9 inlet pipes and 10 outlet pipes are simultaneously opened. After how many hours will the cistern be completely filled?