Question 257395
.15*12+1*x=.5(12+x)
1.8+x=6+.5x
18+10x=60+5x
10x-5x=60-18
5x=42
x=42/5
x=8.4 liters of pure water

check
1.8+8.4=.5(20.4)
10.2=10.2
ok