SOLUTION: Ethan can run twice as fast as he can walk. If after running two hours and then walking two hours Ethan has moved 21 miles, how fast does Ethan run? How fast does he walk? Sol

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Question 1074360: Ethan can run twice as fast as he can walk. If after running two hours and then walking two hours Ethan has moved 21 miles, how fast does Ethan run? How fast does he walk?
Solve for system of equations
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +s+ = his walking speed
+2s+ = his running speed
Let +d+ = his distance walking
+21+-+d+ = his distance running
------------------------------------
Equation for walking:
(1) +d+=+s%2A2+
Equation for running:
(2) +21+-+d+=+2s%2A2+
-----------------------
(1) +d+=+2s+
(2) +21+-+d+=+4s+
Substitute (1) into (2)
(2) +21+-+2s+=+4s+
(2) +6s+=+21+
(2) +s+=+3.5+
and
+2s+=+7+
----------------
He runs 7 mi/hr
He walks 3.5 mi/hr
---------------------
check:
(1) +d+=+2s+
(1) +d+=+7+
and
(2) +21+-+d+=+4s+
(2) +21+-+d+=+14+
(2) +d+=+7+
OK