### The average age of a group of persons going for picnic is 16 years. Twenty new persons with an average age of 15 years join the group on the spot due to which their average age become 15.5 years. The number of persons initially going for picnic is.

A. 5

B. 10

C. 20

D. 30

Let the initial number of persons be x. Then, $\begin{array}{l}\text{⇒ 16x + 20 × 15 = 15.5 (x + 20)}\\ \text{⇒ 0.5x = 10}\\ \text{⇒ x = 20}\end{array}$