Use the Auto Fill feature to fill cells with data that follows a pattern or are based on data in other cells.
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Select one or more cells you want to use as a basis for filling additional cells.
For a series like 1, 2, 3, 4, 5..., type 1 and 2 in the first two cells. For the series 2, 4, 6, 8..., type 2 and 4.
For the series 2, 2, 2, 2..., type 2 in first cell only.
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Drag the fill handle
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If needed, click Auto Fill Options
and choose the option you want.
Source: Microsoft