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guo573874201

1.Which of the following is closest to 10¹⁸⁰ – 10³⁰ ?

A.10²¹⁰
B.10¹⁸⁰
C.10¹⁵⁰
D.10⁹⁰
E.10^{6  找不到答案了 选什么呀  怎么算的 又没有简便的方法啊？}

2.A certain pilot flew 400 miles to City K at an average speed of 350 miles per hour with the wind and made the trip back at an average speed of 250 miles per hour against the wind.Which of the following is closest to the pilot’s average speed, in miles per hour, for the round-trip?
A.280
B.290答案选B  不会算了 算的没这个答案 求过程
C.300
D.310
E.320

HsiaoYuChen

1.Which of the following is closest to 10¹⁸⁰ – 10³⁰ ?

A.10²¹⁰
B.10¹⁸⁰
C.10¹⁵⁰
D.10⁹⁰
E.10^{6  找不到答案了 选什么呀  怎么算的 又没有简便的方法啊？}

2.A certain pilot flew 400 miles to City K at an average speed of 350 miles per hour with the wind and made the trip back at an average speed of 250 miles per hour against the wind.Which of the following is closest to the pilot’s average speed, in miles per hour, for the round-trip?
A.280
B.290答案选B  不会算了 算的没这个答案 求过程
C.300
D.310
E.320

-- by 会员 guo573874201 (2011/9/21 2:32:57)

1.the answer should be B; open to discuss
10^180 是181位數
10^30   是31位數
相減仍是180位數左右. 所以最接近的還是10^180.
例如:10^6-10^2
  1,000,000 (七位數)
-            100 (二位數)
-----------
    999,900  (六位數) is closet  to 10^6

2.
average speed = 來回miles /來回時間
                         = 400*2 / (400/350+400/250)
                         =291.xxx closet to 290 (answer B.)