An equation question for my son.?

Question

There are 60 students in class. Boys contributes $4 & girls contributes $3 to the relief fund. If collected total amount is $220, then find out the number of girls students in the class.

Answer 1

Lets "b" represents boys and "g" represents girls.

Total students in class b+ g = 60--->1

Boys contributes 4 and girls contributes 3 is 4b + 3g = 220--->2

multiply the equation 1 by 3

(b+ g = 60)*3

3b+ 3g = 180--->1

Substract equation 1 with 2

3b+ 3g = 180

4b+ 3g = 220

 (-)  (-)       (-)

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-b          = -40

b = 40

substitute b = 40 in Equation 1

b+ g = 60

40 + g = 60

g = 6o -40

g = 20

There are  20 girl students