Assume we need to add x litters.
So we are going to mix x liters (25% saline) and and 3 liters (10% saline).
In total we are going to have a x+3 litters (of 15% saline).
a. of the x liters, 25/100x=0.25x is saline. so we have 0.25x liters saline.
b. of the 3 liters, (10/100)*3=0.1*3=0.3 liters is saline.
c. of the x+3 liters, (15/100)(x+3)=0.15(x+3)=0.15x + 0.45 liters is saline
equalize:
0.25x + 0.3 = 0.15x + 0.45
0.25x - 0.15x = 0.45 - 0.3
0.1 x =0.15
x=0.15/0.1= 1.5 (liters)
Answer: 1.5 liters
