Problem
From 9:55 PM to 12:00 midnight: from 9:55 to 10:55 is 1 hour, to 11:55 is 2 hours, and 11:55 to 12:00 is 5 more minutes. That is 2 hr 5 min.
Midnight ends the PM half of the day, so it is the natural splitting point.
3.MD.A.1Identify SubproblemsFrom 12:00 midnight to 4:15 AM is simply 4 hours and 15 minutes, since midnight is the start of the AM half.
After midnight the clock reads the AM hours directly, so the second piece is read straight off.
3.MD.A.1Identify SubproblemsAdd the two spans: 2 hr 5 min plus 4 hr 15 min. Minutes: 5 + 15 = 20 min. Hours: 2 + 4 = 6 hr. The total flight time is 6 hr 20 min.
Adding the before-midnight and after-midnight pieces gives the whole elapsed time.
3.MD.A.1Identify SubproblemsThe whole flight time equals the before-midnight span added to the after-midnight span.
Midnight happens partway through the flight, cutting it into two back-to-back stretches — 9:55 PM to midnight, then midnight to 4:15 AM — that touch with no gap and no overlap.
When a stretch of time is split at a moment inside it into pieces that meet with no gap and no overlap, those pieces add back to exactly the whole stretch.
Putting the two spans together, the hours line up with the hours and the minutes line up with the minutes, so you add 2 hr with 4 hr and 5 min with 15 min.
Each span is an amount of hours plus an amount of minutes, and you may shuffle and regroup those amounts to bring the like units side by side before adding.
Changing the order in which you add amounts never changes the total.
Changing which amounts you group together first never changes the total.
When a trip crosses midnight, split the clock at 12:00 into a 'before' and an 'after' piece, then just add them up!