For years, telephone area codes in the united states and canada consisted of a sequence of three digits. the first digit was an integer between 2 and 9, the second digit was either 0 or 1, and the third digit was any integer from 1 to 9. how many area codes starting with a 4 were possible?

The question illustrates permutation and combination.

The total number of area codes starting with 4 is 18.

The selection is as follows:

The total number of selection is:

Hence, 18 area codes starting with 4 are possible.

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The first number had to be 4, so only one option for the first number.

The second number was either 0 or 1, so two options for the second number.

The third number was between 1 and 9, so nine options for the third number.

Multiply each number of options together for total options:

1 x 2 x 9 = 18 different codes could start with 4.

the entire expression should be subtracted from 30. to subtract the quantity, distribute the negative sign, which changes the signs on the terms. so, the –2 changes to positive and gets added to 30 instead of subtracted from it.

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w'x is 0.

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