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: a grocer has a sale of rs. 5420, rs. 5660, rs. 6200, rs. 6350 and rs. 6500 for 5 consecutive months. find the sale he should have in the sixth month, so that he gets an average sale of rs. 6400 ?
Options: a. rs. 5870, b. rs. 8270, c. rs. 6020, d. rs. 6850, e. none of these
[EX A]: b

[EX Q]: Problem: 50 % of a number is more than 40 % of 120 by 180. find the number ?
Options: a. 456, b. 480, c. 420, d. 445, e. 320
[EX A]: a

[EX Q]: Problem: a crow leaves its nest, and flies back and forth from its nest to a nearby ditch to gather worms. the distance between the nest and the ditch is 250 meters. in one and a half hours, the crow manages to bring worms to its nest 15 times. what is the speed of the crow in kilometers per hour ?
Options: a. 1, b. 3, c. 4, d. 5, e. 6
[EX A]:
d