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The formula for the rate is equal to distance ÷ time.

In this case, our distance our work is equal to 1.

∴ Rate = 1/Time

Using this we can then state that:

Rate of A = 1/6
Rate of B = 1/3

The time it would take both of them to  do the work would be equal to 
1/(Rate of A + Rate of B)

Time = 1/(1/6 + 1/3)
         = 1/ (1/2)
         = 2 days

It would take person A and B 2 days to finish the job together.