For the first question think of it this way:
When selecting the first fly the probability it is not of strain B is 12/18.
Now if you are to select another fly that isn't in strain B the chances of that are 11/17 (since the number of non-strain B flies is reduced as well as the total number of flies). The next is 10/16 and the probability for the last fly is 9/15. To get the probability of all these events occurring you multipy them together to get (12*11*10*9)/(18*17*16*15) = 0.162
If you look through your textbook you might see some different notation involving combinatorics and factorials (which you should probably be using) but they accomplish exactly what I have done above.
Don't have time to help you with the others but just try to think along the same lines. And learn how to use the notation in the textbook.