CareerCup

Oops...sorry I was wrong...

The answer is

Another way would be....

If total work is say x units.
person 1 can do x/6 units/day
person 2 can do x/4 units /day

Both can together do (x/6 + x/4) units/day
To complete x units of work both together can do it in x/(x/6+x/4) days which is 2.4 days.

Answer is 1 try.

Since all boxes as mislabelled below is the probability...

Box labelled A + O can contain "A" or "O"
Box labelled O can contain "A" or "A+O"
Box labelled A can contain "O" or "A+O"

If you pick "A" from box labelled A+O,

Box labelled A + O, contains "A"
Box labelled O, contains "A+O"
Box labelled A, contains "O"

If you pick "O" from box labelled A+O,

Box labelled A + O contains "O"
Box labelled O contains "A"
Box labelled A contains "A+O"