His average speed for the first trip is 43 mph
Since Brian's average speed is x and he travels the first part for 215 miles in time t.
So, his distance d = xt
Also, the travels 75 more miles at a speed of 15 mph.
Since distance = speed time, the time he takes to cover this distance is
t' = distance/speed = 75 mi/15 mph = 5 hours
Since he covers both distances in the same time, we have that t = t' = 5 hours.
So, his average speed for the first trip x = d/t
= 215 mi/5 h
= 43 mph
His average speed for the first trip is 43 mph
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