Step-by-step explanation:
Model:
s = slices
x = pizzas ordered
s = .8(.45x) + .12(.55x)
Now plugin 20 for x and you will get the total number of slices.
s = .8(.45x) + .12(.55x)
s = .8(.45(20)) + .12(.55(20))
s = 204 slices
Note, we could make this model different and smaller but I chose this method because it demonstrates what is going on with the word problem