Question

Ling lives 2 miles from school. It took him 15 minutes to bike from school to home. The first half of the distance he biked at a speed of 12 mph. What was his speed for the remaining distance? What was his average speed?

His speed for remaining distance was___mph and his average speed was___mph.
so he biked 1 mile at 12 mph and the other 1 mile at x mph

1/12 + 1/x = 1/4 ---> 15 min = 1/4 hr
times 12x, (don't like fractions)
x + 12 = 3x
x = 6
so his speed for the second part was 6 mph

check:
time at 12 mph = 1/12 hr = 5 minute
time at 6 mph = 1/6 hr = 10 min
for a total of 15 min

Average speed = total distance/total time
= 2/(1/4) = 8 mph

do you notice that 8 is NOT the average of 12 and 6 ???
hi

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