Divide 50 into two parts so that the sum of their reciprocals is   $${1\over 12} $$

Find the standard deviation of the following data (rounded off to two decimal places). 5, 3, 4, 7 

The value $${7+3\sqrt {5}\over3+\sqrt { 5}}-{7-3\sqrt {5}\over3-\sqrt { 5}}$$ lies between

The sum of the present ages of a father and son is 52 years. Four years hence, the son's age will be $${1\over 4}$$that of the father. What will be the ratio of the ages of the son and father, 10 years from now?

Ratio of the monthly income of A to B is 4:3 and the ratio of the monthly income of B to C is 2:1. If the difference between the C’s monthly income and his savings is Rs.12000 and the saving’s of C is half of his expenditure, then what is the average of the income of A, B and C?

For which value of x the seven-digit number 8439x53 is divisible by 99?

Find two such numbers whose middle proportional is 18 and third proportional is 144.

The average salary, per head, of all the workers of an institution is 60. The average salary of 12 officers is 400; the average salary, per head, of the rest is 56. The total number of workers in the institution is -