There are 4 clocks in a room. One gains a minute every hour. Another loses a minute every hour. One runs backward at normal speed. The fourth always keeps the correct time.
At 7:03 today, they all showed the same time, which was correct. When will this happen again?
Answer
In 60 days.
If one clock gains a minute a day (or loses, the math will be the same), it will gain 24 minutes the first day, 48 minutes by the end of the second, and 120 minutes after 5 days. This means in ten days it will gain 4 hours and in 20 days, 8 hours. This times 3, to make it 24 hours, will require 60 days.
The other clock running backward will tell the same time as the normal clock every 24 hours, so it really doesn’t present a problem for the solution of the puzzle.