单选题 A receptionist typed 8 memos, each either 1 or 2 pages long. If the receptionist typed 12 pages in all, how many of the memos had 2 pages?
  • (A) 1
  • (B) 2
  • (C) 3
  • (D) 4
  • (E) 5


【正确答案】 D
【答案解析】To solve this problem, set x = the number of 1-page memos typed. The number of 2-page memos typed will equal 8-x. Add the total number of pages typed from 1-page memos, x, to the total number of pages typed from 2-page memos: x+2(8-x)=12. Solve for x:
x+2(8-x)=12
x+16-2x=12
x=4
Because the question asks for how many 2-page memos the receptionist typed, substitute x=4 into 8-x to yield an answer of 4.