I always love a controversial one, so I was interested to look at their rounding (to nearest minute) in Q27c. They did as I would have, and rounded 57.14 minutes to 57 minutes. The Matrix solutions however rounded up to the nearest minute, 58 minutes, so as to ensure the rounded solution meet or exceeded the minimum energy requirement.
As time is not an inherently discrete variable, I interpreted the rounding as simply avoiding unnecessary apparent precision in a real world problem with inherently approximate modeling. Matrix however have interpreted the time in minutes as a discrete variable, and looked at the minimum value of that discrete variable needed to satisfy the condition.
Interesting situation, I wonder if both answers (57 and 58) will be accepted?