One person can do a certain job in six days, a second person can do the same job in two days, and a third person can do this job in three days. how many days will they take to do the job together? a. 1 day b. 2 days c. 6 days

Let the three person be represented by X, Y and Z.
X can do 1/6 of the job in 1 day
Y can do 1/2 of the job in 1 day and 
Z can do 1/3 of the job in 1 day
Together, these 3 people will do: 1/6 + 1/2 + 1/3 = 1 of the job in one day
Thus, the three people will do the job in one day.
The correct option is A