Answer:
20.640
Explanation:
√426 ≈ 20.6397674406
The digit in the ten-thousandths place (bold) is more than 4 so 1 needs to be added to the thousandths place to round the number. Then, 39 thousandths becomes 40 thousandths, and the rounded number is ...
... 20.640
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Since you are asked to round to thousandths, you must show the thousandths digit of the rounded number, even though it is zero. This identifies the number as having precision to thousandths, even though it can be expressed using only hundredths.