Correct Answer - Option 3 : 20

**Given:**

Initial average = 16.75 years.

20 new persons with an average age of 13.25 years joined

New average = 15 yrs.

**Formula: **

Average of n numbers a1, a2, ………. an = (a1 + a2 + ... + an)/n

**Calculation:**

Let the no. of person initially going for the trip be x.

Total age of x persons = x × 16.75

Total age of 20 persons = 13.25 × 20 = 265

Total age of all the persons = 15 × (x + 20)

From the question

16.75x + 265 = 15(x + 20)

⇒ 16.75x + 265 = 15x + 300

⇒ 1.75x = 35

⇒ x = 20

**∴ Initial number of person is 20**