During rehearsal for the Founders' Day choir program, the director, Mrs. Mazurek, tried 3 different configurations, each using all the choir members at the rehearsal. One configuration was to have only rows of 12, one was to have only rows of 15, and one was to have only rows of 20. None of these configurations worked because for each, the last row had 1 person less than the other rows. What was the least of the possible numbers of choir members at the rehearsal?

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Finding the multiples of each:

$$ 12: 12, 24, 36, 48, 60... $$ $$ 15: 15, 30, 45, 60... $$ $$ 20: 20, 40, 60... $$

At 60 students, each seating configuration will have 1 extra seat in the last row.