Peter and Kristin are to read 20 fantasy novels each in a week. Peter can read three times as fast as Kristin. If Peter reads one book in 18 hours, how long will Kristin read half of her books? Give the rationale and then the answer.
Let's think step by step. Since Peter reads three times as fast as Kristin, Kristin will take 18 * 3 = 54 hours to read one book. Kristin reads half of her books, a total of 20 / 2 = 10 books. The time Kristin will take to read half of her books is 10 * 54 = 540.. The answer is: 540.

Two friends are racing three miles. The first one runs it in 21 minutes. The second one runs it in 24 minutes. If they keep up the same pace, how long combined will it take for them to run 5 miles each? Give the rationale and then the answer.
Let's think step by step. The first one runs a mile in 7 minutes because 21 / 3 = 7. The second runs a mile in 8 minutes because 24 / 3 = 8. The first one would finish 5 miles in 35 minutes because 5 x 7 = 35. The second one would finish in 40 minutes because 5 x 8 = 40. It will take 75 minutes in total because 35 + 40 = 75.. The answer is: 75.

John buys thank you cards. He decides to buy them for people who got him Christmas and birthday presents. He sent 20 for Christmas gifts and 15 for birthday gifts. If each card cost $2 how much did he spend on cards? Give the rationale and then the answer.
Let's think step by step. He bought 20 + 15 = 35 cards. So he spent 35 * 2 = $70.. The answer is: 70.