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.
--------
Question: Problem: 30 pens and 75 pencils were purchased for 750. if the average price of a pencil was 2.00, find the average price of a pen.
Options: a. 20, b. 11, c. 12, d. 13, e. 14

Answer: a

Question: Problem: the total marks obtained by a student in mathematics and physics is 50 and his score in chemistry is 20 marks more than that in physics. find the average marks scored in mathamatics and chemistry together.
Options: a. 15, b. 25, c. 35, d. 45, e. 55

Answer: c

Question: Problem: by selling an article at rs. 900, a shopkeeper makes a profit of 25 %. at what price should he sell the article so as to make a loss of 25 % ?
Options: a. s. 600, b. s. 480, c. s. 500, d. s. 540, e. s. 550

Answer:
d