Q: Jack wants to send thank you cards to everyone who got him a gift for his wedding. He sent out 200 invitations. 90% of people RSVPed but only 80% of people who RSVPed actually showed up. Then 10 people who showed up didn't get a gift. How many thank you cards does he need?
A: He got 200 * .9 = 180 RSVP. Of those only 180 * .8 = 144 people showed up. So only 144 - 10 = 134 got him gifts and hence need thank you cards. The answer is 134.

Q: There are 20 students in a class. Only one-fourth of the students stayed in the classroom while the rest went to the playground. Of those who went to the playground, one-third are boys. How many girls are there on the playground from this class?
A: Out of the 20 students, 20 x 1 / 4 = 5 students stayed in the classroom. While 20 - 5 = 15 students went to the playground. Out of these 15 students, 15 x 1 / 3 = 5 are boys. Therefore, 15 - 5 = 10 girl students from this class are in the playground. The answer is 10.

Q: Will and Henry go fishing in a river. Will catches 16 catfish and 10 eels. Henry challenges himself to catch 3 trout for every catfish Will catches. Due to environmental concerns, Henry decides to return half his catch after meeting his own challenge. How many fishes do they have altogether now?
A:
Will caught 16 + 10 = 26 fishes. Henry caught 3 trout for each catfish Will caught so since Will caught 16 catfish, Henry caught 16 * 3 = 48 trout. Half of 48 trout is (1 / 2) * 48 = 24. Henry returned 24 trout leaving him with 48 - 24 = 24 trout. They now have a total of 26 + 24 = 50 fishes. The answer is 50.