Anyway, we've got 1/5 of a cup of OJ. which means we have 4/5 left to go to get to a full cup. We have to

**split that 4/5 up 3 ways**, because the question tells us that the mix we use to fill the cup is split evenly in 3 parts. Forget about the other parts; we only care about the OJ.

To find how much more OJ we add, first convert 4/5 to 12/15. That's nice and divisible by 3. What's one third of 12/15? Because that's how much more OJ we add.

**We add 4/15 of a cup more OJ to our mix.**

We started with 1/5 of a cup, which I'm going to convert to 3/15 now so we're adding with common denominators. We're adding 4/15 of a cup more OJ to our mix that started with 3/15 of a cup. We end up with 7/15 of a cup of OJ.