Dmitri was working on a mixture whose weight comprise of 90% liquid and 10% solid. The total weight of the mixture is 20 pound. After a while, Dmitri noticed that some of the liquid evaporated and now the liquid comprise just 50% of the weight.

What is the weight of the mixture now?

4 Pounds

90% of liquid = 18 pounds
10% of solid = 2 pounds

Earlier 10% of solid is now contributing 50% weight = 2pounds
Therefore weight of liquid is also contributing 50% weight = 2pounds

Sum = 4pounds