If a ten digit telephone number is of the form (XYZ)-ABC-DEFG. The seven digit phone numbers that are possible with these restrictions is: 25,200,000 phone numbers.
XYZ=4 possible area code=C'4
'A' number 2 through 8=C'7
B and C= C'10 C'9
D,E,F, and G= C'10 C'10 C'10 C'10
Hence,
Possible ways=C'4 C'7 C'10 C'9 C'10 C'10 C'10 C'10
Possible ways=4×7×10×9×10×10×10×10
Possible ways=25,200,000 phone numbers
Therefore If a ten digit telephone number is of the form (XYZ)-ABC-DEFG. The seven digit phone numbers that are possible with these restrictions is: 25,200,000 phone numbers.
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