What is the weight ratio of water and 30% ammonia solution to get a 12% solution?

Given:
ω1 (NH3) = 30%
ω2 (NH3) = 12%

To find:
m (H2O): m1 solution (NH3) -?

1) Since water is a pure solution, then ω (H2O) = 0%;
2) To solve such a problem, the most optimal would be to use the cross rule (mixing rule, diagonal model of the “Pearson envelope”)
3) Calculate the ratio by mass of water and the original ammonia solution:
m (H2O): m1 solution (NH3) = 18: 12 = 3: 2.

Answer: The ratio of water and initial ammonia solution (by weight) is 3: 2.