Let x be the speed for the first two miles and x+.5 be the speed for the last 8 miles. Since D=rt, 2x+8x+.5is the time it took Janet to do the 10 mile race. If she took her bike the whole way, her time would be 10x+.5. Since that was two minutes faster than her real time, 2x+8x+.5-10x+.5=2. Solving for s you get s=.5. The question asks for the average speed for the whole race so, 2.5+8.5+.5=10a where a is the average speed. A= 56 miles per minute.
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