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?

In a right triangle for an acute angle x, if sin x = $\frac{3}{7}$, then find the value of cosx.

In a class of 60 students, there are 35 boys. The average weight of boys is 42 kg and the average weight of the full class is 46.5 kg. Find the average weight of girls in the class.