From the condition, you have these two equations

     G +  B = 40      (1)

    5G - 4B = 65      (2)

where G is the number of girls and B is the number of boys.


From equation (1), express  B = 40-G and substitute it into equation (2), replacing B there. You will get

    5G - 4*(40-G) = 65

    5G + 4G       = 65 + 160

     9G           = 225

      G           = 225/9 = 25.


ANSWER.  25 girls and the rest, 40-25 = 15 are boys.