When minutes elapse on a correct clock, minutes register on clock (fast), and only minutes register on clock (slow). If later in the day, clock reads and clock reads , what was the correct time when the two clocks were originally set?
Source: NCTM Mathematics Teacher, February 2006
Clock reads while clock reads implies that clock is minutes ahead of clock . Since clock is ahead of clock minutes per hour, hours must have passed from the time they were set
hours minutes/hour = minutes
hours before is . Clock is minutes fast per hour. We need to make clock go back an extra minutes to pass hours exactly.
Alternatively, from the perspective of clock hours before is . Clock is slow minutes per hour. We need to spring it forward minutes to pass hours exactly.
is the correct time when the two clocks were originally set.