
When you ask "What number is halfway between 21 and 643?" we assume you mean the number in the exact middle of the two numbers on a number line, as illustrated below, where X=21 and Y=643.

To find the number halfway between 21 and 643, we use the following formula:
(X + Y)/2 = Halfway Between
When we enter 21 and 643 into our formula, we get:
(21 + 643)/2 = 332
Thus, the number halfway between 21 and 643 is 332.
To check that our answer is correct, we subtract 21 from 332 and subtract 332 from 643 and compare the two numbers:
332 - 21 = 311
643 - 332 = 311
As you can see, the halfway number is 311 higher than 21 and 311 lower than 643. Therefore, our halfway between answer of 332 above is correct.
Halfway Between
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Halfway Between 21 and 644
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