Maths quiz

class 3 

A takes 12 hours to do1  work 

so in 1 hour A will do 1/12 ( one by 12) work ...

similarily B takes 8  hours to do1  work 

so in 1 hour A will do 1/8 ( one by 8) work ...

so togather in one hour A and B do (1/12+ 1/8 ) = 5/24 hours...

so 5/24 work is done in 1 hours

therefore , to do one work they will need 1/(5/24) = 24/5 = 4.8 hours..  

A's 1 hour's work = 1/12

B's 1 hour's work = 1/8

(A + B)'s 1 hour's work = (1/12+1/8) = 5/24

Both A & B together will finish the work in 24/5 hours (1*1/(5/24))

The Work has already been done.....

Why will they do it again????