We bought a total of 80 books at the store. Math books cost $4 and history books cost $5. The total price was $390. How many math books did we buy?	Choose the correct answer:
A) 10  B) 20  C) 30  D) 40  E) 50

Rationale: "m + h = 80 h = 80 - m 4 m + 5 h = 390 4 m + 5 * ( 80 - m ) = 390 m = 10 the answer is a ."
Answer: A