Question

in a school 5/8 of the students were girls if the no of girls is 120 more than  that of  the boys what is the strength of the school  how many boys are there

Answer 1

Let there be "x" boys in the school. Then, as per question, there are "x+120" girls. 
So, total number of students = boys + girls
                                              = (x) + (x+120)
                                              = 2x + 120
We know that 5/8 of the students are girls. Hence:
                                       girls = $\frac{5}{8}$ (2x + 120)
                         But, we know that : 
                                       girls = x + 120
 Using this :
                          x + 120 = $\frac{5}{8}$ ( 2x + 120)
                        8(x + 120) = 5(2x + 120)
                        8x + 960 = 10x + 600
                         Therefore :
                                          2x = 360 
                                     and x = 180
                        Therefore, x + 120 = 300

There are :
1. 180 boys
2. 300 girls 
3. 480 students in all

Answer 2

Ans:-

Let there be "x" boys in the school. Then, as per question, there are "x+120" girls. 

So, total number of students = boys + girls

                                              = (x) + (x+120)

                                              = 2x + 120

We know that 5/8 of the students are girls. Hence:

                                       girls =  (2x + 120)

                         But, we know that : 

                                       girls = x + 120

 Using this :

                          x + 120 =  ( 2x + 120)

                        8(x + 120) = 5(2x + 120)

                        8x + 960 = 10x + 600

                         Therefore :

                                          2x = 360 

                                     and x = 180

                        Therefore, x + 120 = 300

There are :

1. 180 boys

2. 300 girls 

3. 480 students in all