Social Security numbers are of the form XYZ - MN - ABCD
Part A:
The first three digits XYZ are referred to as the area number
X is a number 1 - 7 (1, 2, 3, 4, 5, 6, 7)
That means we have 7 choices for X
Y and Z can be any number 0 - 9 (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
That means we have 10 choices for Y and also 10 choices for Z
So, the number of possible ways are
But we know that the area code 666 has never been used.
So we need to subtract this one from the total.
Therefore, the total number of possible area codes is 699.