At 6 am, some taxis are waiting in the line at an airport taxi stand. In each of the next three hours, first exactly half of the taxis in the line leave with passengers, and then 6 more taxis join the line. No other taxi joins or leaves. At 9 am there are 15 taxis in the line. How many taxis were in the line at 6 am?
24
78
72
36
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