Question 66165
.5x+.1(60-x)=60*.25
.5x+6-.1x=15
.4x=15-6
.4x=9
x=9/.4
x=22.5 liters of 50% & (60-22.5)=37.5 liters of 10% are needed 
proof
.5*22.5+.1(60-22.5)=60*.25
11.25+.1(37.5)=15
11.25+3.75=15
15=15