SOLUTION: A cruise ship left Port 37 and traveled south. A submarine left nine hours later traveling 18 mph faster in an effort to catch up to it. After five hours the submarine finally caug

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Question 937773: A cruise ship left Port 37 and traveled south. A submarine left nine hours later traveling 18 mph faster in an effort to catch up to it. After five hours the submarine finally caught up. Find the submarine's average speed.
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +s+ = the speed of the cruise ship in mi/hr
+9s+ = the cruise ship's head start in miles
+s+%2B+18+ = the speed of the submarine in mi/hr
Let +d+ = distance in miles that the submarine travels
until it catches the cruise ship
--------------------------
Start measuring time in hrs when submarine leaves
Cruise ship's equation:
(1) +d+-+9s+=+s%2A5+
Submarine's equation:
(2) +d+=+%28+s+%2B+18+%29%2A5+
---------------------
Substitute (2) into (1)
(1) +%28+s+%2B+18+%29%2A5+-+9s+=+s%2A5+
(1) +5s+%2B+90+-+9s+=+5s+
(1) +9s+=+90+
(1) +s+=+10+
and
+s+%2B+18+=+10+%2B+18+
+s+%2B+18+=+28+
The submarine's average speed was 28 mi/hr
---------------------------------
check:
(2) +d+=+%28+10+%2B+18+%29%2A5+
(2) +d+=+28%2A5+
(2) +d+=+140+ mi
and
(1) +d+-+9s+=+s%2A5+
(1) +d+-+9%2A10+=+10%2A5+
(1) +d+=+90+%2B+50+
(1) +d+=+140+ mi
OK