Given: Average of weight of 4 students = 70 Formula used: Average = Sum of all observation/Number of observation Calculation: Sum of the weight of 4 students =70×4=280 Sum of the weight of 3 students =60×3=180 Weight of student who left the group =280–180=100kg Formula used: Quantity removed = [Change in number of quantities × Original average] - [Change in average × Final number of quantities] =[1×70–(−10)×3] ⇒100kg