Two. The '7' telling how many tens we've got, and the '2' telling how many ones.
231.57 has five significant figures/numbers. All the numbers in 231.57 are significant.
18
The figure 18.03 has a total of four significant numbers
If you are counting the numbers 17, 18, 19, ..., 72, you will count (72 - 17) + 1 = 56 numbers (note that I added 1 because the set is inclusive, i.e. it includes 17 and 72).
72, 72, and 72.
there are many (infinite) ways but just one is 72=2*(31+5)
There are infinitely many pairs. Two possibilities are: 1 * 72 10 * 7.2
Because 10.00400 is 10.004, this has five significant digits (numbers).
Three. All nonzero numbers are significant, and any zeros in between significant numbers are significant.
There are 5 significant figures.
1 significant figure.
5 significant figures.