Answer:
Let
x = the number of pepperoni added to the pizza.
y = the number of mushrooms added to the pizza.
z = the number of onions added to the pizza.
u = the number of olives added to the pizza.
Twice as many onions as pepperoni
z = 2*x
Half as many mushrooms as olives
y = u/2
There are 39 individual toppings on the pizza
x + y + z + u = 39
We are dealing with a system of equation with 4 unknowns and only three equations.
2*x - z = 0
y - u/2 = 0
x + y + z + u = 39
This means that there are an infinite number of solutions for your problem
For example, here is one solution
x = 1
z = 2
1 + y + 2 + (2y) = 39
3y = 36
y = 12
u = 24