Andrew M. answered • 04/03/17

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Mathematics - Algebra a Specialty / F.I.T. Grad - B.S. w/Honors

Barber 1 does 1/24 heads per sec

Barber 2 does 1/18 heads per sec

Together they do 1/24 + 1/18 heads per sec

(1/24 + 1/18)s = 21

(3/72 + 4/72)s = 21

(7/72)s = 21

s = 21(72/7)

s = 3(72)

s = 216 seconds to do 21 heads