What's the answer and how do i solve it

If 20% of a number is 35, what is 50% of that number?

The number if marbles of different colors stored in a hat is listed below: 4 red marbles 10 green marbles 7 blue marbles. without looking in the hat, dan takes out a marble at random. he replaces the marble and then takes out another marble from the hat. what is the probability that dan takes out a blue marble in both draws?

How can constraints be used to model a real-world situation?