The total 7-digit telephone numbers has the form abc-xxxx which are possible are 3,720,087
It is given that,
A 7-digit telephone number has the form = abc - xxxx
Also, x, b, c can have any digit between 1-9
And, a is restricted to between 3-9
We need to find the possible number of combinations of a 7-digit telephone number having the form abc-xxxx
Then, number of digits that can be placed at the place of 'a' = 7 type of digits (3 to 9)
Number of digits that can be placed at the place of x, b and c= 9 type of digits (1 to 9)
Suppose, we put a number from 3 to 9 at 'a', so total possibilities are 7
Similarly, at 'b' we put a number from 1-9, so total possibilities are 9 and so on so forth
Thus total 7-digit telephone numbers which have the possible combinations can be
= 7*9*9*9*9*9*9
= 3,720,087
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