To start, it's always easiest to turn percentages into decimals- so let's do that first:
10% Weak solution = .1
30% Strong solution = .3
15% Medium solution = .15
Next we need to assign a value to the amount of each solution we have or need.
Weak solution = x (because we don't yet know how much we need, we give it a variable)
Strong solution= 20-x (because once we determine x and know there are 20 total liters, we can simply subtract to figure out the remainder)
Medium solution= 20 (because in total we need 20 liters)
Now we create the formula to solve for x.
.1x+.3(20-x)=.15(20)
x=15
So you would need 15 liters of the weaker 10% solution and 5 liters of the stronger 30% solution. This makes sense b/c the average 15 is much lower than 30 so we'd expect to need much more of the weaker solution.
