Explanation
Let number of boys be "x" and number of percent 60%. If 60% are boys in a school then, 40% are girls in a school. Total amount of students = (x + 120)
60% of (x+120) is boys (x)
\(\frac{60}{100} \times (x + 120) = x\)
\(\frac{3}{5} \times (x+120) = x\)
\(3 x+360=5 x\)
\(2 x=360\)
\(x=180\)
So, number of boys are 180