SOLUTION: Coach ran 4 km in 30 minutes. He ran the first 2 km at a constant speed that was twice the constant speed at which he ran the last 2 km. Coach ran the 3rd km at a speed of ? km

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Question 413255: Coach ran 4 km in 30 minutes. He ran the
first 2 km at a constant speed that was twice
the constant speed at which he ran the last 2 km.
Coach ran the 3rd km at a speed of ? km/hr.
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
1st 2 km:
(1) 2+=+r%5B1%5D%2At%5B1%5D
2nd 2 km:
(2) 2+=+.5%2Ar%5B1%5D%2At%5B2%5D
(3) t%5B1%5D+%2B+t%5B2%5D+=+.5 hr
---------------------
(3) t%5B2%5D+=+.5+-+t%5B1%5D
and
(1) t%5B1%5D+=+2%2Fr%5B1%5D
and
(2) 2+=+.5%2Ar%5B1%5D%2A%28.5+-+t%5B1%5D%29
(2) 2+=+.5%2Ar%5B1%5D%2A%28.5+-+2%2Fr%5B1%5D%29
(2) 2+=+.25%2Ar%5B1%5D+-+1%29
(2) .25r%5B1%5D+=+3
(2) r%5B1%5D+=+12
and
r%5B2%5D+=+.5r%5B1%5D
r%5B2%5D+=+6 km/hr
Coach ran the 3rd km at 6 km/hr
check answer:
(1) t%5B1%5D+=+2%2Fr%5B1%5D
(1) t%5B1%5D+=+2%2F12
(1) t%5B1%5D+=+1%2F6
(3) t%5B1%5D+%2B+t%5B2%5D+=+.5
(3) t%5B2%5D+=+3%2F6+-+1%2F6
(3) t%5B2%5D+=+1%2F3
t%5B1%5D+%2B+t%5B2%5D+=+1%2F6+%2B+2%2F6
t%5B1%5D+%2B+t%5B2%5D+=+.5 hr
OK