There are 15 students in a class. Their average weight is 40 kg. When one student leaves the class, the average weight is 39.5 kg. What is the weight of the student who left the class (where kg means kilogram)?

Number of students in the class = 15

Average weight of the students = 40 kg

Average weight after one student leaves = 39.5 kg

The total weight of all students is equal to the average weight multiplied by the number of students. When a student leaves, the total weight decreases by the weight of that student, and the average changes accordingly.

​$$\text{Total weight of all students} = \text{Average weight} \times \text{Number of students}$$ ​

Weight of the student who left = Initial total weight − New total weight

x = (40 × 15) – (39.5 × 14)
x = 47kg Ans

Alternate method →
Average dropped
x = 40 + (40 – 39.5) × 15
x = 47 ​​