Question 924793
let x be the amount of 25% solution to add to the 42 cc of pure water, then we have the following table:
x  .25  .25x
42 .00    0
x+42 .20  .20(x+42)
from the last column we have the following formula
.25x = .20(x+42)
.25x = .20x + 8.40
.05x = 8.40
x = 168 cc of 25% solution to be added to the 42 cc of pure water