A boy runs for 10 mins at a uniform speed for 9 km/h. At what speed should he run for further 20 mins, so that his average speed is 12km/h?
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Answer:
9/10+x/20=12
(18+10x)/20=12
18+10x=240
10x=240-18
10x=222
x=222/10
x=22.2
22.2km/h
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