The average weight of some students in a class was 69.5 kg. When 10 students of average weight 68 kg joined the class, and 6 students of average weight 60 kg left the class, it was noted that the average weight of the newgroup of students increased by 2 kg. How many students are there in the class now?

21

10 students  join the group ,

Sum of weight of 10 students  = 10 × 68 = 680 kg

6 students leave the group ,

Sum of weight of 6 students  = 6 × 60 = 360 kg

Difference in weight = 680 - 360 = 320 kg

Let initial number of person = N

According to question ,

N × 69.5 + 320 = ( N + 4 ) × 71.5

69.5N + 320 = 71.5N + 286

2N = 34

N = 17

So , Number of students in class now = N + 4 = 17 + 4  = 21