CPhill while reading your answer i just figured out another method which could give the same right answer !

check this out :

let 7 and 8 be 7x and 8x.

ATQ ; 7x +8x = 45

=15x =45

x= 45/15

x=3

therefore ,

7x = 7*3 = 21

8x=8*3=24

isnt that correct

by the way thanks for another formula but dont mind i find this one a lil hard