490 to 2 s.f.
Note that when the significance is uncertain, you must specify it (s.f. is an abbreviation of significant figures).
You can also underline the last significant digit:
490
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8 digits just count the amount of numbers.
493 is not prime. 493 = 17 * 29
493 + 8 = 501
328 935 493 196
1 493/1000