Question 1095515
<br>Distilled water is 0% acid.<br>
And don't let the "draining off" confuse you.  In the end, you will be mixing some amount x liters of 0% acid with another amount (5-x) of 28% acid to end up with 5 liters of 20% acid.<br>
Re-post if that is not enough to get you going.