2 Adults to 1 Minor means twice as many Adults as Minors
A + M = membership
A + 24 = 2M If you add 24 to A there will be twice as many A as M (let's divide this equation by 2)
1/2 A + 12 = M and re-arrange
12 = M - 1/2 A
Also given
M-x = 1/2 A re-arrange
M-1/2 A = x then look at the RED equation above
x=12