2015 · Q96
The average weight of students in a class is 43 kg. Four new students are admitted to the class whose weights are 42 kg, 36-5 kg, 39 kg and 42-5 kg respectively. Now the average weight of the students of the class is 42-5 kg. The number of students in the beginning was
Elementary MathematicsAlgebra
- A10
- B15
- C20
- D25
Explanation
This is a weighted-average problem. Let the original class have n students averaging 43 kg (total 43n kg). The four newcomers weigh 42 + 36.5 + 39 + 42.5 = 160 kg. The new average is 42.5: (43n + 160)/(n + 4) = 42.5, giving 43n + 160 = 42.5n + 170, so 0.5n = 10 and n = 20. Checking: (860 + 160)/24 = 1020/24 = 42.5, as required. Hence option (c).