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.

[EX Q]: Problem: dhoni bought 5 toys with the average cost of $ 10. if david also buys the sixth toy with the price of $ 16, what is the average ( arithmetic mean ) price of those 6 toys ?
Options: a. $ 10, b. $ 7, c. $ 8.5, d. $ 9, e. $ 11
[EX A]: e

[EX Q]: Problem: when jessica withdrew $ 200 from her bank account, her account balance decreased by 2 / 5. if she deposits an amount equal to 2 / 3 of the remaining balance, what will be the final balance in her bank account ?
Options: a. 300, b. 375, c. 400, d. 500, e. 575
[EX A]: d

[EX Q]: Problem: a man swims downstream 30 km and upstream 20 km taking 5 hours each time, what is the speed of the man in still water ?
Options: a. 7, b. 8, c. 2, d. 5, e. 4
[EX A]:
d