#1) Think of this as an experiment and list four possible outcomes.
Answer: There are 3 possible outcomes. The first one is for the condition that the first digit cannot be zero and the digits can be repeated: 9*10*10 = 900. The second outcome is if the digits cannot be repeated: 9*9*8 = 648. The last possible outcome is if zero can be the first digit an the numbers can be repeated: 10*10*10 = 1000. Also the probability of Mr. Jones and Mrs. Smith of selecting the same PIN is 1/900.
I hope it helps, Regards.