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.

Problem: when positive integer n is divided by 3, the remainder is 1. when n is divided by 7, the remainder is 5. what is the smallest positive integer k such that k + n is a multiple of 21 ?
Options: a. 2, b. 4, c. 6, d. 8, e. 10
a

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
e

Problem: a boat crossed a lake from north to east at the speed of 7 km / h, entered a river and covered twice as much distance going upstream at 6 km / h. it then turned around and stopped at the south shore of the lake. if it averaged 3.8 km / h that day, what was its approximate downstream speed ?
Options: a. 4, b. 5, c. 6, d. 7, e. 8
e