This is similar to constant total concept, except that one total is twice of the other.
1. get the total of the ratio of the two group.
2. Work out mulitples of both the school hall and the AVA room, so that the total of the school hall is twice of AVA. In this case it’s easy, mulitple of 2 for school hall will work.
AVA room, B :G : Total => 7 : 1 : 8
School Hall, B : G : Total => 5 : 3 : 8 => 10 : 6 :16
Highlight in red now share common units (u).
a) School hall boys : AVA Girls => 10 : 1 ##
B) See attached picture.