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: dropbox charges $ 3.10 for first 1 / 5 of a month for 500 gb storage, plus $ 0.40 for each additional 1 / 5 of a month. what would dropbox charge for a data storage of 500 gb for 8 months ?
Options: a. $ 15.60, b. $ 16.00, c. $ 17.80, d. $ 18.70, e. $ 19.10
d

Problem: the lcm of two numbers is 2310 and hcf is 55. if one of the numbers is 210. then what is the other number ?
Options: a. 715, b. 825, c. 330, d. 605, e. 465
d

Problem: in what ratio must water be mixed with milk to gain 16 2 / 3 % on selling the mixture at cost price ?
Options: a. 1: 9, b. 1: 1, c. 1: 2, d. 1: 6, e. 1: 5
d