Calculating Probability: Sample of Workers & Shifts | Probability Question

[TitaniumX]

Question
If a production facility employs 20 workers on the day shift, 15 workers on the swing shift, and 10 workers on the graveyard shift.

d) What is the probabily that drawing 6 slips (without replacement) that at least one of the shifts will be unrepresented in the sample of workers.

This is part D of a multi-part question from Probability class. I'm not sure how to solve it, but I'm guessing you can use the equation "1-P(that all shifts is represented)"... but how do you calculate the probability of that? Thank you

 

[Vanadium 50]

This is a thinking question, not a calculating question. Even this may be too much of a hint.