How many minutes is it before 12at noon if 90 minutes ago it was four times as many minutes past 9 a. m. ?

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问题 How many minutes is it before 12at noon if 90 minutes ago it was four times as many minutes past 9 a. m. ?

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答案72

解析 (设是中午12点以前x分钟。根据题意可得此方程式：x=4[12×60一(90+x)一9×60]解得X=72)

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