How Many Seven-Digit Telephone Numbers Are Possible If the First

question 177

Essay

How many seven-digit telephone numbers are possible if the first digit must be nonzero?
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How many international direct-dialing numbers are possible if each number consists of a four-digit area code (the first digit of which must be nonzero) and a number of the type described in part (a)?
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Definitions:

Allele

A member of a pair of genes.

Homozygous Trait

A genetic condition where an individual inherits identical alleles for a particular gene from both parents.

Heterozygous Trait

A genetic condition where an individual has two different alleles for a particular gene, one inherited from each parent.

Dizygotic Twins

Also known as fraternal twins, born from two separate eggs fertilized by two different sperm, sharing about 50% of their DNA.