==> During the first 5 minutes, the plane's speed increased linearly from zero to 600 mph.
Its AVERAGE speed during the first 5 minutes was 300 mph.
Traveling at an average speed of 300 mph for 5 minutes (1/12 of an hour), it covered (300 x 1/12) = 25 miles, in the first 5 minutes.
==> From 5 minutes to 25 minutes ... an interval of 20 minutes (1/3 hour) ... the plane traveled at a constant 600 mph.
Traveling at a speed of 600 mph for 1/3 of an hour, it covered
(600 x 1/3) = 200 miles, during the time from 5 minutes to 25 minutes.
==> On the whole graph, the plane traveled
..... 25 miles, from zero to 5 minutes
.. 200 miles, from 5 to 25 minutes
Total distance: 225 miles in the 25 minutes shown on the graph.