题目内容
阅读理解
阅读下列短文,从每题所给的四个选项(A、B、C、D)中选出最佳选项。
It is five o'clock. What was the time 7 hours ago? The situation corresponds to (符合) the subtraction (减去) problem 5-7=? In clock 12.
One way to answer the problem is to count the hours on an ordinary clock, but this time, count counterclockwise (逆时针). If the time is five o'clock now, then seven hours ago it was 10 o'clock. Thus, 5-7=10 in clock 12. How can you check result?
In ordinary subtraction, you can check your answer by setting up the related addition (加法) problem. For example, to check 20-13=7, ask “Is seven the number that must be added to 13 to produce 20”?
20-13=7 can be changed into20=7+13.
To check 5-20=-15, ask“Is 5=20-15 true?” This method may be used to check a subtraction problem in clock 12.If 5-7=10 is true.
Then 5=7+10 has to be true. Indeed, 5 is equal to 7+10 in clack 12.Thus, 5-7=10 in clock. So we can know.
In clock arithmetic, as well as in ordinary arithmetic.
a-b=c means a=b+c
1.If it is 8 o'clock, now, what was the time 9 hours ago?
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2.Which is correct in clock 12?
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3.Tom started at 6 o'clock. Ten hours later he reached London and immediately met Jack. Jack said he had arrived 4 hours before, that is, at ________ o'clock.
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4.The unit in clock arithmetic is ________ that in ordinary arithmetic.
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