10 liters of the 15% alcohol drink contain 0.15*10 liters of pure alcohol.
If you add W liters of water, the mixture will contain the same 0.15*10 liters of alcohol, but the total liquid volume will be (10+W) liters.
You need to find W under the condition
= 0.05,
which is the condition that the mixture is 5% alcohol.
OK. You just have an equation.
To solve it, multiply both sides by (10+W). You will get
0.15*10 = 0.05*(10+W),
1.5 = 0.5 + 0.05W ====> 0.05W = 1.5 - 0.5 = 1 ====> W = = 20.
Answer. 20 liters of water must be added.
Check. = = 0.05 = 5%.
