During a sale at a bookstore, joseph buys a book at full price. he is given a 50 percent discount on a second book of equal or less value. if joseph buys two books that have a full price of $15 and $10, by what percent is the total cost of the two books reduced during the sale? (a) 5% (b) 20% (c) 25% (d) 30% (e) 50%

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.