Since you know what the first three numbers are, it boils down to how many possible ways the last 4 digits can be.
You can't apply nCn or nPn because there's no constraint that the digits must be unique.
How can I solve this problem without using the fundamental counting principle?
The 7-digit phone numbers in a city all have 661 as the first three digits. How many phone numbers are possible?
I was told this is a permutation NOT a combination word problem.
In that case, can I apply nPn or nPr. If not, why not?