If the solution is 20% glucose then it's 80% water.

Let the amount of 30% glucose solution used be x ounces. Note that this solution is 70% water.

Then the amount of pure water used is (7 - x) ounces and it's obviously 100% water.

You want the result to be 80% water. So solve:

(70/100)x + (100/100)(7 - x) = (80/100)(7), that is,

0.7x + (7 - x) = (0.8)(7).