Question 1157274
x=16% sol in l
60-x= 12% sol 
60*0.13=7.8 liter of pure acid
.16x+.12(60-x)=7.8
.16x+7.2-.12x=7.8
0.04x=0.6
=15 liters of 16%
D

Notice that 13% is 1/4 of the way from 12 to 16%
That means 3/4 of the acid is weighted for 12% of 45 l, leaving 15 l for 16%