In a Physics test, the average marks of girls in a class is 40. If the average of first 20 girls is 50 and that of the remaining girls is 20, then total number of girls in the class is :
Answer & explanation
Correct answer: option 1
Average marks of total girls = 40
Assume Total girls = x
Total marks of all girls = 40x
Average marks of First 20 girls = 50
Total marks of 20 girls = 50*20 = 1000
Remaining girls = x-20
Average marks of remaining girls = 20
Total marks of remaining girls = 20(x-20) = 20x - 400
Thus, 20x-400 + 1000 = 40x
20x = 600
x = 30
The correct answer is option (1) : 30