4.The arithmetic mean of certain observations is 35 If each number is increased by 5, what will be the arithmetic mean ?

The correct answer is (b).

The arithmetic mean of a set of numbers is the sum of the numbers divided by the number of numbers. If each number in a set is increased by a constant, the arithmetic mean of the set will also increase by that constant.

In this case, the arithmetic mean of the set is 35. If each number is increased by 5, the new arithmetic mean will be 35 + 5 = 40.