Good question, Arturo. The Weighted Average method, from the Problem Solving lesson (lesson 10) works really well here. If solution A is 20 parts salt and solution B is 80 parts salt, but the resulting mixture is 50 parts salt, you could draw out a number line with:
20 (A) 50 (combined) 80 (B)
The distance from A to the average is 30, and from B to the average is 30, so the distances would be in the ratio of 30:30, or 1:1.
So, we know that we'll need equal parts A and B, so the correct answer choice is 4:4.
Now, one last step - we need to make sure that it's possible to have equal amounts of each add up to 50. Because we have 30 of A and 60 of B, it does work - we could have 25 of each.
If you did this algebraically, you could say that:
.2a + .8b = .5(50) (20% of A + 80% of B = 50% of 50, using the amount of salt as the percentage)
a + b = 50 (the combination of a and b will add up to 50)
a = 50 - b (to substitute for a in the first equation)
.2 (50 - b) + .8b = .5(50)
I'd multiply everything by 10 to get rid of the decimals and make it cleaner:
2(50 - b) + 8b = 250
100 - 2b + 8b = 250
100 + 6b = 250
6b = 150
b = 25
So there are 25 ounces of b, and 50 ounces total, so a must also be 25 ounces, giving us a ratio of 1:1, which also works out to 4:4.