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 sum of money at simple interest amounts to rs. 720 after 2 years and to rs. 1020 after a further period of 5 years. the sum is :
Options: a. 400, b. 500, c. 600, d. 700, e. 800
[EX A]: c

[EX Q]: Problem: bruce and anne can clean their house in 4 hours working together at their respective constant rates. if anne ’ s speed were doubled, they could clean their house in 3 hours working at their respective rates. how many w hours does it currently take anne to clean the house on her own ?
Options: a. 6, b. 7, c. 8, d. 12, e. 14
[EX A]: d

[EX Q]: Problem: 50 litres of diesel is required to travel 600 km using a 800 cc engine. if the volume of diesel required to cover a distance varies directly as the capacity of the engine, then how many litres of diesel is required to travel 800 km using 1200 cc engine ?
Options: a. 80 litres, b. 90 litres, c. 100 litres, d. 170 litres, e. none of these
[EX A]:
c