Solution X contains 40% salt and 60% water by weights. Solution Y contains 25% salt and 75% water. If a mixture of X and Y contains 30% salt, what percent of the weight of this mixture is solution X?

A 10%

B 33.3%

C 40%

D 42.5%

Correct Answer: Option B

Let x g be the weight of solution X in 100 g of the mixture.
Then, the weight of solution Y in 100 g of the mixture = (100 - x) g

Mass of salt in the mixture :
x (40%) + (100 - x) (25%) = 100 (30%)
⇒ 40x + 2500 - 25x = 3000
⇒ 15x = 500
⇒ x = 500/15 = 33.33

Hence, there are 33.33% of solution X in the mixture.