SOLUTION: At 10:30 am two planes leave Houston, one flying east at 560 km/h and the other flying west at 640 km/h. At what time will they be 2100 km apart?

Question 648179: At 10:30 am two planes leave Houston, one flying east at 560 km/h and the other flying west at 640 km/h. At what time will they be 2100 km apart?
Found 2 solutions by josmiceli, DrBeeee:
Answer by josmiceli(19441) (Show Source): You can put this solution on YOUR website!
Just think of one plane as standing still and
the other moving away at the sum of their speeds.

hrs
hr = 45 min
10:30 + 1:45 = 12:15
They will be 2100 km at 12:15 PM
check answer:
plane #1

plane #2

OK
Answer by DrBeeee(684) (Show Source): You can put this solution on YOUR website!
Let t = the amount of time the planes travel
Let d1 = distance travelled by east bound plane
Let d2 = distance travelled by the west bound plane
Formula to use
(1) d = r*t, where d is the distance travelled, km, in time t, hr, when travelling at a rate of r km/hr
For the first plane
(2) d1 = 560*t
For the second plane
(3) d2 = 640*t
The total distance travelled or distance apart after t hr is
(4) d1 + d2 = 2100
Substituting (2) and (3) into (4) yields
(5) 560*t + 640*t = 2100 or
(6) 1200*t = 2100 or
(7) t = 7/4
Answer: The planes will be 2100km apart at 12:15pm.
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