How many liters of a 25 percent saline solution must be added to 3 liters of a 10 percent saline solution to obtain a 15 percent saline solution?

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