4. At 8:00 my watch was four minutes slow. However, it gains time at a constant rate and at 16:00 on the same day it was six minutes fast. At what time did it show the correct time?
A 9:10
B 10:11
C 11:12
D $12: 13$
E 13:14
Solution
C

In the 8 hours, or 480 minutes, from 8:00 to 16:00, my watch gains 10 minutes. Since it is 4 minutes slow at 8:00, it will show the correct time in $\frac{4}{10}$ of 480 minutes after 8:00. Hence it will show the correct time 192 minutes after 8:00, which is $11: 12$.