填空题 How many minutes is it before 12 noon if 90 minutes ago it was four times as many minutes past 9 am?

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【正确答案】 1、72    
【答案解析】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-(90+x)-9×60]，解得x =72。