A boat running upstream takes 7 hours and 40 minutes to cover a certain distance while it takes 5 hours to cover the same distance downstream. What is the ratio between the speed of the current and speed of the boat respectively? 

A polygon has 9 sides, what is its interior angle?

Simplify : $$\sqrt{[(12.1)^2-(8.1)^2]÷[(0.25)^2+(0.25)(19.95)]} $$

The length of the common chord of two circles of radii 30 cm and 40 cm whose centers are 50 cm apart is (in cm) 

Find the LCM of the fractions $${2\over 5}$$, $${3\over 7}$$ and $${4\over 9}$$

If 80 Persons can finish a work in 16 days by working 6 hours a day, the number of hours a day should 64 persons work to finish that same job in 15 days is:

How many pairs of twin primes are there between the integers 1 to 100? 

The interest compounded annually on a sum of Rs. 56000 at a rate of 7% p.a. after 3 years is what percentage of the simple interest on a sum of Rs. 72000 at a rate of 9% p.a. after 5 years?