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 :

30

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