If Danielle ran a race at a constant speed, at what time did she finish?
(1) Danielle started the race at 8:00 a.m.
No info about the speed, or time.
Not sufficient
(2) At 9:30 a.m. Danielle was halfway through the race, and at 10:00 a.m., she was 2/3 of the way through the race.
This means that in 30 minutes (10:00-9:30) she runs \frac{2}{3}-\frac{1}{2}=\frac{1}{6} of the race.
At 10:00 she has \frac{1}{3} left to run or \frac{2}{6} and, according to the upper calculus, she will run it in 1 hour (1/6 every 30 minutes).
She will finish at 11:00 am.
Sufficient
B
(1) Danielle started the race at 8:00 a.m.
No info about the speed, or time.
Not sufficient
(2) At 9:30 a.m. Danielle was halfway through the race, and at 10:00 a.m., she was 2/3 of the way through the race.
This means that in 30 minutes (10:00-9:30) she runs \frac{2}{3}-\frac{1}{2}=\frac{1}{6} of the race.
At 10:00 she has \frac{1}{3} left to run or \frac{2}{6} and, according to the upper calculus, she will run it in 1 hour (1/6 every 30 minutes).
She will finish at 11:00 am.
Sufficient
B