Marine barber #1 can shave on head every 24 seconds. Marine barber #2 can shave on head every 18 seconds. Together, how many seconds will it take them to shave-

Question

21 heads.   Please help me!!

Answer 1

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