Multiple choice

One watch is $1$ minute slow at $1$ pm on Tuesday and $2$ minutes fast at $1$ am on Friday. When did it show that correct time?

  1. $5.00$ am on Wednesday
  2. $9.00$ am on Wednesday
  3. $5.00$ pm on Wednesday
  4. $9.00$ pm on Wednesday
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The watch gains 3 minutes in 60 hours (from Tuesday 1 pm to Friday 1 am). To show the correct time, it must gain 1 minute. 1 minute is gained in 20 hours. Adding 20 hours to Tuesday 1 pm results in Wednesday 9 am.

AI explanation

The watch goes from being 1 minute slow to 2 minutes fast, meaning it changes by a total of 3 minutes relative to the correct time. The time difference between 1 pm on Tuesday and 1 am on Friday is exactly 60 hours. Using the uniform error method, the watch shows the correct time after it covers the initial 1 minute error, which takes 1 divided by 3 of the total time span, meaning 20 hours after 1 pm on Tuesday. Adding 20 hours to 1 pm on Tuesday brings the time to 9:00 am on Wednesday.

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