It's the other way round. Think of it this way.
D is bigger than Q. You need to multiply Q by 2 to get the equal amount of D.
D = 2Q
N = 3Q
Q + N + D = 54
From there, put everything in Q.
Q + 3Q + 2Q = 54
Solve for Q. The rest will come easily.
Think in terms of "what should I do to X to make it equal to Y?"
It helped me a lot, maybe it can hel you too!