Question

Destiny has 4 liters of 49% acid. How much water should she add to make a solution that is 14% acid?

Answer 1

the key here is that the amount of acid doesn't change.
add x liters of water, you have the total amount of liquid (x+4) liters
4*49%=(4+x)*14%
0.14x=1.96-0.56
x=10
she needs to add 10 liters of water

Answer 2

Destiny has 4*.49 = 1.96 L of acid. In order for the 1.96 L of acid to be 14% of the solution, there must be 1.96/.14 = 14 L of solution.

Destiny must add 14 L -4 L = 10 L of water to make the desired solution.