The answer depends on how many significant figures. To one sig fig it is 20.
It is 6.7 when rounded to 2 significant figures
You didn't say how many significant figures. But, rounded to six significant figures it would be 5206.00
At least 1 and at most 7. It could be 3,999,999.9 rounded to 7 significant figures; It could be 3,999,999 rounded to 6 significant figures; It could be 4,000,015 rounded to 5 significant figures; It could be 4,000,429 rounded to 4 significant figures; It could be 3,999,999 rounded to 3 significant figures; It could be 4,049,999 rounded to 2 significant figures; It could be 4,492,467 rounded to 1 significant figure.
0.00721004 rounded into three significant figures is 0.00721
9,976,200 rounded to two significant figures is 10,000,000.
16.275 rounded to 3 significant figures is 16.3
0.000709999 rounded to 3 significant figures is 0.000710
44.981 rounded to three significant figures is 45.0
0.00721004 rounded to three significant figures is 0.00721
79,966.92 rounded to three significant figures is 8.00e4
1,399.54 rounded to three significant figures is 1.40e3
1,430 rounded to 2 significant figures is 1,400