0.25 of an hour is 15 minutes.
1 hour 40 minutes 42 seconds is between 1 hour 45 minutes and 1 hour 30 minutes and
1 hour 40 minutes 42 seconds is closer to 1 hour 45 minutes than to 1 hour 30 minutes.
THEREFORE, 1 hour 40 minutes 42 seconds rounded to the nearest 0.25 hour is 1 hour 45 minutes, or 1 of an hour. ANSWER