Measure Time by Hourglass

Problem:
Using only a four-minute hourglass and a seven-minute hourglass, what time can be measured with the process not taking longer than the time we are trying to measure.
A)8 min
B)11 min
C)6 min
D)9 min

Answer:A,B,D

Solution Approach:
 0:00 - Start both at the same time.
4:00 - The four minute glass runs out. Flip the four minute glass.
7:00 - The seven minute glass runs out. Flip the seven minute glass.
8:00 - The four minute glass runs out; the seven minute glass has been running for one minute. Flip the seven minute glass.
9:00 - The seven minute glass runs out.
11:00 -When both of them complete