【正确答案】 E

【答案解析】By TACTIC El, if the average of 5 numbers is 60, their sum is 5 × 60 = 300. The 4 given numbers add up to 100 (10 + 20 + 30 + 40), so the fifth one, x, is 200 (300 -100).
**Use TACTIC 1, Chapter 11. Test the choices, starting with 100. The average of 10, 20, 30, 40, and 100 is 40. That's too small. Eliminate Choices A, B, and C, and try a larger number— 150 or 200:200 works.