There are 4 signifigant figures in 19.83
Three of them.
1 in 1 km. 1, 2 3 or 4 in 1000 m since it is ambiguous.
1.0*103
1.0 x 10^3
5 of them.
There are 4 signifigant figures in 19.83
Six.
4
0.990
Three of them.
1 significant figure.
1000 and another 1000 1000 1000
Rounded to three significant figures, 1000 square feet is equal to 0.0230 acres.
Oh, dude, there are four significant figures in 0.03000. The zeros at the beginning are just placeholders, so they count as significant figures. It's like having four friends at a party, even if two of them are just hanging out in the corner not saying much.
(4.73*1000*0.568)+1.61 = 2688.25 meaning that it has 6 significant figures
Just calling it a "1000-sided shape" is fine. It is too much trouble to search for fancy name for figures with many sides.