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11 Feb 2020, 01:36
Kudos
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D
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
65%
(hard)
Question Stats:
66% (03:02) correct
34%
(03:13)
wrong
based on 185
sessions
History
Date
Time
Result
Not Attempted Yet
Ted and Robin start from the same point at 7 AM and drive in opposite directions. Ted doubles his speed after every 90 minutes whereas Robin reduces her speed by half after every 120 minutes. If Ted starts driving at a speed of 10 kilometers/hour and Robin starts driving at a speed of 120 kilometers/hour, how far in kilometers will they be from one another at 1 PM?
A) 195
B) 485
C) 525
D) 645
E) 675
A) 195
B) 485
C) 525
D) 645
E) 675
11 Feb 2020, 07:39
Kudos
Bookmarks
They'll drive for a total of 6 hours. Ted's speed doubles every 1.5 hours, so he will drive at four different speeds: 10, 20, 40 and 80 km/hr. Since he's driving at each speed for an equal amount of time, his average speed for the whole trip will just be the straight average of those four speeds, so will be 150/4 km/hr. So in the six hours, he drives (6)(150/4) = (3)(75) = 225 km.
Robin's speed halves every 2 hours, so she will drive at three different speeds: 120, 60 and 30 km/hr. Again her average speed will be the average of those three speeds, so will be 210/3 = 70 km/hr. In six hours, she thus drives 420 km.
Since they're driving away from each other, starting from the same point, the distance separating them will just be the sum of the distances each travels, so will be 225 + 420 = 645 km.
Robin's speed halves every 2 hours, so she will drive at three different speeds: 120, 60 and 30 km/hr. Again her average speed will be the average of those three speeds, so will be 210/3 = 70 km/hr. In six hours, she thus drives 420 km.
Since they're driving away from each other, starting from the same point, the distance separating them will just be the sum of the distances each travels, so will be 225 + 420 = 645 km.
17 Feb 2020, 04:24
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Raxit85
From 7 AM to 8:30 AM, Ted’s speed is 10 kmph; from 8:30 AM to 10 AM, his speed is 20 kmph; from 10 AM to 11:30 AM, his speed is 40 kmph; finally, from 11:30 AM to 1 PM, his speed is 80 kmph. Therefore, from 7 AM to 1 PM, Ted drives:
10 x 1.5 + 20 x 1.5 + 40 x 1.5 + 80 x 1.5
(10 + 20 + 40 + 80) x 1.5
150 x 1.5
225 km
Likewise, from 7 AM to 9 AM, Robin’s speed is 120 kmph; from 9 AM to 11 AM, her speed is 60 kmph; and finally, from 11 AM to 1 PM, her speed is 30 kmph. Therefore, from 7 AM to 1 PM, Robin drives:
120 x 2 + 60 x 2 + 30 x 2
(120 + 60 + 30) x 2
210 x 2
420 km
Therefore, together they drive a total of 225 + 420 = 645 km.
Answer: D
