Hope this helps!
There are 3 images with full explanation
8x - amount of boys
7x- amount of girls
8x + 7x = 195
15x= 195 ( divide by 15)
x= 13
8x = 8 x 13 = 104
7x = 7 x 13= 91
there are 104 boys, and 91 girls.
The number beside the letter . for example 6x, 6 would be the variable