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
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.
The figure 18.03 has a total of four significant numbers
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 an infinite number of real numbers that multiply to get 72.
The numbers are: 12 and 72
Two significant figures.