Q: In Dana's senior high school class there were 200 students. 60% of the students were female, and 50% of the females were brunettes. If 50% of the female brunettes were under 5 feet tall, then how many female brunettes in Dana's high school senior class were under 5 feet tall?
A: In Dana's senior class, the 60% of 200 students is 0.6 * 200 = 120 female students. Fifty percent of the females, or 0.5 * 120 = 60 of the female students were brunettes. If 50% of the female brunettes were under 5 feet tall, then there were 60 * 0.5 = 30 female brunettes under five feet tall in Dana's senior class. The answer is 30.

Q: It takes Derek 9 minutes to walk a mile if he doesn't have to walk with his brother. If he has to take his brother it takes 12 minutes to walk a mile. How many minutes longer would it take him to walk 20 miles if he had to take his brother?
A: Alone it would take him 9 * 20 = 180 minutes. With his brother, it would take him 12 * 20 = 240 minutes. 240 - 180 = 60 more minutes. The answer is 60.

Q: Cleo and Ignacio placed 30 of their marbles in a jar on a Monday. They took 3/5 of the marbles from the jar the next day and divided them equally. On the third day, Cleo took 1/2 of the marbles remaining in the jars. How many marbles did Cleo have on the third day?
A:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
On the first day, Cleo and Ignacio took 3 / 5 * 30 marbles = 18 marbles from the jar. When they shared the marbles, each got 18 marbles / 2 people = 9 marbles / person. The total number of marbles remaining in the jar is 30 marbles - 18 marbles = 12 marbles. On the third day, Cleo took 1 / 2 * 12 marbles = 6 marbles from the jar. The total number of marbles she has on the third day is 6 marbles + 9 marbles = 15 marbles. The answer is 15.