Seeing it at first, you might think about using simple proportions, but logic tells you that they should complete it in a lower period.
This is a problem which requires inverse proportions.
It is mainly in the form
where y is the time and x is the number of assistants.
Using the first piece of information, you can obtain the value of the constant k.
Then, you use that constant k with the new asked number of assistants and solve for x, the time that they take. What do you get? :)