How many pounds of water must be evaporated from 50 pounds of a 3% salt solution so that the remaining solution will be 5% salt?

Having a hard time understanding this question (see attachment).

The price of a visit to the dentist is calculated according to the formula 50+100n50+100n where nn is the number of cavities the dentist finds. on your last visit to the dentist, 22 cavities were found. what was the cost of your visit?

In a singing competition, there are 150 participants. at the end of each round, 40% of the participants are eliminated. how many participants are left after n rounds?