There is a bag with only red marbles and blue marbles. The probability of randomly choosing a red marble is

5

8

.

There are 56 marbles in total in the bag and each is equally likely to be chosen.

Work out how many red marbles there must be.

Ariana starts with 100 milligrams of a radioactive substance. the amount of the substance decreases by 20% each week for a number of weeks, w. the expression 100(1−0.2)w finds the amount of radioactive substance remaining after w weeks. which statement about this expression is true? a) it is the difference between the initial amount and the percent decrease. b) it is the difference between the initial amount and the decay factor after w weeks. c) it is the initial amount raised to the decay factor after w weeks. d) it is the product of the initial amount and the decay factor after w weeks.