Question 1183647
Dialing phone numbers works on the basis of the correct order of the digits. 

Hence, the phone number XXXXXXX135 would be different from the number XXXXXXX315, or the number XXXXXXX531.

In other words, the probability that John dials the correct phone number on the first try is {{{1/P(5,3) = highlight(1/60)}}} ~ 0.0167, to 4 d.p.