It is 4am and i am really sleepy, but let us do another simple problem. in a lot of 20 tires, 3 are defective. if you picked 4 tires at random, what is the probability that you will get 1 defective tire?

First, you pick is not defective with a probability of 17/20, the second with a probability of 16/19, the third 15/18, and the fourth 14/17.

So the probability of picking no defective tyres is:
 (14*15*16*17)/(17*18*19*20)=168/342=84/171

So the probability of picking at least one defective tyre is 87/171 (just over half).