49 of them.
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The factor pairs of 21 are:1, 213, 7Therefore, the number 21 has two pairs of positive integer factors.
There are infinitely many pairs of numbers including many pairs of positive integers. An easy pair to remember is 1 and 675.
3 pairs of whole numbers add up to 7: 1 +6, 2+5, 3+4 (0+7 also equals 7, but zero is not a whole number)
five pairs
Four unique pairs.