Question 206208
This is the same as selecting 3 customers without replacement and finding the probability that each one will make a purchase.

The probability of the 1st customer making a purchase is {{{18/20}}}
But for the 2nd customer, after having selected the 1st customer without replacement, the probability is {{{17/19}}} because you remove one customer that has made a purchase.
Do the same for the 3rd customer: {{{16/18}}}

Now multiply them together: {{{(18/20)*(17/19)*(16/18)}}}={{{68/95}}}