Question

The first three digits of a university telephone exchange are 452. If all the sequences of the remaining four digits are equally likely, what is the probability that a randomly selected university phone number contains seven distinct digits?

Answer 1

so, I like to draw a diagram for this type

Answer 2

you have seven boxes, each can only be filled with one thing

Answer 3

the first 3 only have 4,5,2

Answer 4

the next four, can be any number including repeats

Answer 5

so, can you draw the diagram?