Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

[Q]: Problem: in one hour, a boat goes 19 km / hr along the stream and 9 km / hr against the stream. the speed of the boat in still water ( in km / hr ) is :
Options: a. 12 kmph, b. 13 kmph, c. 14 kmph, d. 15 kmph, e. 16 kmph
[A]: c


[Q]: Problem: if a - b = 6 and a 2 + b 2 = 50, find the value of ab.
Options: a. a. 7, b. b. 12, c. c. 15, d. d. 18, e. e. 20
[A]: a


[Q]: Problem: a certain store sells all maps at one price and all books at another price. on monday the store sold 10 maps and 8 books for a total of $ 22.00, and on tuesday the store sold 20 maps and 15 books for a total of $ 60.00. at this store, how much less does a map sell for than a book ?
Options: a. $ - 5.4, b. $ 0.50, c. $ 0.75, d. $ 1.00, e. $ l. 25
[A]:
a