This question is pretty straight forwars, asking for the number of hours it took for helen to drive to her parents house.
(1) tells us that the total distance(D) over the total time (T) = 72km/hr. However, since we don't know anything else, we cannot determine how long it took her. For example the total distance could be 72 km, in which case it would have taken her 1 hour. Or the total distance could be 216 km, in which case it would have taken her 3 hours.
(2) so if her average speed was 8 hours greater, then it would have taken her 1 hour less. You can put it into an equation like so D/(T-1)=(D/T)-8. However, there are multiple times for this to work.
If we look at both together, however we know that D/T=72km/hr and that D/(T-1)=80 at this point we can stop and see that we can solve it, so you can just pick C, but I will finish the problem for you anyway :)
D=72T, so 72T/(T-1)=80
80T-80=72T ---> 8T=80, T=10
Hope this helps!
Eliza, GMAT Tutor