Find the value of x that makes a||b

Which ratio is equivalent to 10: 85

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?

Use sigma notation to represent the following series for 9 terms.