Tanya has 2 red marbles, 5 blue marbles, and 9 green marbles. Which expression can be used to find the percent of Tanya's marbles that are blue?

Iam so stuck. what is the worked out answer for this question?

The brain volumes (cm cubed) of 50 brains vary from a low of 904 cm cubed to a high of 1488 cm cubed. use the range rule of thumb to estimate the standard deviation s and compare the result to the exact standard deviation of 175.5 cm cubed, assuming the estimate is accurate if it is within 15 cm cubed

F(n + 1) = f(n) – 8. if f(1) = 100, what is f(6)?