Find the average of all prime numbers from 1 to 20?

How may digit are required to write numbers from 1 to 50?

Find the value of $$ {1\over{2×5}}+{1\over {5×8}}+{1\over {8 × 11}}+......{1\over {23 × 26}}=?$$

The greatest number less than 1500, which is divisible by both 16 and 18, is:

If the arithmetic means of 3a and 4b is greater than 50, and a is twice b, then the smallest possible integer value of a is: 

Product of digits of a 2-digit number is 27. If we add  54 to the number, the new number obtained is a number formed by interchange of the digits. Find the number.

What least value which should be added to 1812 to make it divisible 7,11 and 14? 

Divide 32 into two parts such that the sum of the square of the parts is 674 what is the value of the parts?