A and B ran, at their respective constant rates, a race of 480 m. In the first heat, A gives B a head start of 48 m and beats him by 1/10th of a minute. In the second heat, A gives B a head start of 144 m and is beaten by 1/30th of a minute. What is B's speed in m/s?
(A) 12
(B) 14
(C) 16
(D) 18
(E) 20
OA:A
Source: gmatclub.forum
A marathon-Hard Q.
This topic has expert replies
Let the speed of A be "y" m/s and the speed of B be "x" m/s. We can write equations based on the results of the two heats as:
(480-48)/x - 480/y = 60/10 - - - - (1)
480/y - (480-144)/x = 60/30 - - - - (2)
Add (1) & (2). You get,
(480-48-480+144)/x = 8 or 96/x = 8 i.e., x is 12 m/s
(480-48)/x - 480/y = 60/10 - - - - (1)
480/y - (480-144)/x = 60/30 - - - - (2)
Add (1) & (2). You get,
(480-48-480+144)/x = 8 or 96/x = 8 i.e., x is 12 m/s
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Here's how I solved it,
Distance = 480m
1st heat - A runs 480m whereas B runs only 432m because of the 48m headstart,
SpeedA = 480/ta1; SpeedB = 432/tb1
Here ta1 = tb1 - 6 (since A beats B by 1/10th of a min = 6secs)
2nd heat - A runs 480m whereas B runs only 336m because of the 144m headstart,
SpeedA = 480/ta2; SpeedB = 432/tb2
Here ta1 = tb2 + 2 (since A loses to B by 1/30th of a min = 2secs)
Both A and B run with the same speed in both races, so their speeds can be equated from heat1 and heat2
So, A's speed in terms of B would be,
480/(432/SpeedB)-6 1st heat
and 480/(336/SpeedB)+2 2nd heat
Equate these two and solve,
8*SpeedB = 96
SpeedB = 12m/sec
Ans A
Distance = 480m
1st heat - A runs 480m whereas B runs only 432m because of the 48m headstart,
SpeedA = 480/ta1; SpeedB = 432/tb1
Here ta1 = tb1 - 6 (since A beats B by 1/10th of a min = 6secs)
2nd heat - A runs 480m whereas B runs only 336m because of the 144m headstart,
SpeedA = 480/ta2; SpeedB = 432/tb2
Here ta1 = tb2 + 2 (since A loses to B by 1/30th of a min = 2secs)
Both A and B run with the same speed in both races, so their speeds can be equated from heat1 and heat2
So, A's speed in terms of B would be,
480/(432/SpeedB)-6 1st heat
and 480/(336/SpeedB)+2 2nd heat
Equate these two and solve,
8*SpeedB = 96
SpeedB = 12m/sec
Ans A
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Hi I used a bit unconventional method. Although you can use the method suggested by Mitch and other guys, I'll explain you what I did.
Note the numbers here. First one is 48 and the next one is 144. Both these numbers are multiple of 12.
So I plugged 12 In the question and found A's speed from equation 1. Then I plugged these two values in the second equation and hey! Both they satisfied the condition
Note the numbers here. First one is 48 and the next one is 144. Both these numbers are multiple of 12.
So I plugged 12 In the question and found A's speed from equation 1. Then I plugged these two values in the second equation and hey! Both they satisfied the condition