Question

One person can do a certain job in twenty-one minutes, and another person can do the same job in twenty-eight minutes. How many minutes will it take them to do the job together?
6 min
12 min
24 min

Answer 1

C1=J/21min
C2=J/28min
t(C1+C2)=J->
->t=(1/(1/28+1/21))->t=12min