In the first problem: 100.0 has four significant figures but 10.0 only has three. The answer would be properly written as 10.0 (three significant figures).
In the second problem: 250 is ambiguous (it could be two or three significant figures) but it doesn't really matter since 5.0 only has two significant figures. The answer would be written as 50. (decimal point not period)
250 / 5.0 = 50.
When dividing, the sig figs are determined with the number that has the lowest (10.0 has 3 sig figs, so the answer also has 3 sig figs)
Same thing as before. 5.0 has 2 sig figs, so the answer, 50., also has 2 sig figs (the period after the number indicated that the 0 is significant).
Hope this helps.
thanks man =)