Adding a multiple of 10 to a whole number through 1,000 is understood as adding tens within the place-value system: the ones digit remains unchanged while the tens and, when needed, hundreds are recomposed (for example, 270 + 60 = 330). Mental reasoning may involve counting by tens or combining place-value units, including crossing a hundred; the focus is on whole-number sums in this range rather than larger-number generalizations or formal algorithms.
When you add a multiple of , count by tens. The ones digit stays the same because you are adding tens, not ones.
Example: Find .
Therefore,
You can also think of it as tens plus tens, which makes tens, or .
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