The average marks of 50 students in a class are 52. The average marks of boys and girls in that subject are 50 and 55, respectively. The number of boys in the class is :

30

Let the no. of boys in the class be X

The no. of girls in the class be (50 - X).

According to the questions,

55 × (50 - X) + (50 × X) = (50 × 52)

 2750 - 55X + 50X = 2600

150 = 5X

 X = 30

So, the number of boys = 30

∴ The number of boys in the class is 30.