SOLUTION: Two aeroplanes pass each other in flight while travelling in opposite directions.Each aeroplane continues on its flight for 45 minutes after which the aeroplanes are 840 km apart.

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Question 889402: Two aeroplanes pass each other in flight while travelling in opposite directions.Each aeroplane continues on its flight for 45 minutes after which the aeroplanes are 840 km apart. The speed of the first aeroplane is 3/4 of the speed of the other aeroplane. Calculate the average speed of each aeroplane.
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +d+ = the distance in km the faster plane travels
after they pass each other
+840+-+d+ km = the distance in km the slower plane
travels after they pass each other
Let +s+ = the average speed of the faster plane in km/hr
+%283%2F4%29%2As+ = the average speed of the slower plane
-----------------------------------------------------
Slower plane's equation:
(1) +840+-+d+=+%283%2F4%29%2As+%2A%2845%2F60%29+
Faster plane's equation:
(2) +d+=+s%2A%2845%2F60%29+
-----------------------
Substitute (2) into (1)
(1) +840+-+s%2A%2845%2F60%29+=+%283%2F4%29%2As+%2A%2845%2F60%29+
(1) +840+-+s%2A%283%2F4%29+=+%283%2F4%29%2As+%2A%283%2F4%29+
(1) +840+-+s%2A%283%2F4%29+=+%289%2F16%29%2As+
Multiply both sides by +16+
(1) +16%2A840+-+12s+=+9s+
(1) +21s+=+13440+
(1) +s+=+640+
and
+%283%2F4%29%2A640+=+480+
The average speed of the faster plane is 640 km/hr
The average speed of the slower plane is 480 km/hr
check answer:
(1) +840+-+d+=+%283%2F4%29%2As+%2A%283%2F4%29+
(1) +840+-+d+=+%283%2F4%29%2A640+%2A%283%2F4%29+
(1) +840+-+d+=+%28+9%2F16+%29%2A640+
(1) +840+-+d+=+360+
(1) +d+=+480+ km
(2) +d+=+s%2A%283%2F4%29+
(2) +d+=+640%2A%283%2F4%29+
(2) +d+=+480+ km
OK