Question

the ratio of girls and boys in class is 1:3 set an equation between the student of a class and boys and then draw it's graph also find the number of boys in a class of boys in a class of 40 students from the graph

Answer 1

... ANSWER IS SIMPLEST WAY...
1x + 3x = 40
4x = 40
x = 40/4
x = 10

So, 1x = 10 [Number of girls in class]
3x = 3×10 = 30 [Number of boys in class]