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Student A does exactly 9 minutes of homework nightly. Student B manages to do 11 minutes. As not to exceed these limits, they work together, using sandclocks that measure 5 and 7 minutes respectively. With this help, they manage to time their work exactly. How?Set 5-minute clock (F) and 7-minutes clock (S) simultaneously. When F runs out, turn it upside down. When S runs out, turn both upside down. When F is exhausted, 9 minutes have passed. Then turn up S, which has then only 2 minutes left.