At the beginning of the year, the ratio of the number of girls to the total number of students was 3: 7.

At the beginning of the year, the ratio of the number of girls to the total number of students was 3: 7. During the year, 4 girls arrived and the class of boys and girls became the same. How many boys are in the class?

Let x be the number of boys in the class.
y is the number of girls in the class.
Let’s compose a system of equations:
y / (x + y) = 3/7; (one)
y + 4 = x;
y = x – 4; (2)
Substitute y from (2) into (1):
(x – 4) / (x + x – 4) = 3/7;
(x – 4) / (2x – 4) = 3/7;
7 (x – 4) = 3 (2x – 4);
7x – 28 = 6x – 12;
x = 16 boys.
Answer: There are 16 boys in the class.