Julia has eaten one-quarters of the pizza. So three-quarter of the pizza remain.  Therefore the required ratio is $$\frac{3}{4}:\frac{1}{4}=3:1$$. 
Julia has eaten one-quarters of the pizza.  What is the ratio of the number of pizza that remain to the number Julia has eaten? 

1) $$1:3$$ 
2) $$3:1 $$ 
3) $$1:4 $$ 
4) $$4:1$$ 
5) $$4:3$$ 