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Bunuel
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Bunuel
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D/24 -1/12 = D/30 +1/15

By solving the equation we get D =18

How do we get the equation?

Here we are making equation for usual time.


In the first case 5 minutes late from usual time. So we have to subtract the extra time, which is 5 min or 5/60 or 1/12 hr, to get the usual time

Similarly we have to add 4/60 or 1/15 hr in the 2nd case to get the usual time.
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I did this similar to gmatser1, but a bit different (in hours).

24 km/hr -> 1/12 hrs late
30 km/hr -> 1/15 hrs early

D=24*(t+1/12)=30*(t-1/15)
solve for t=2/3 hours (how long it should take at the "typical" speed - not given)

go back to the equation above to find D:
D=24*(2/3+1/12)=24*(9/12)= 18km
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Answer: A

Initial speed = 24 km/hr
Later speed = (5/4) *24 km/hr = 30 km/hr

Now, Difference of time in hrs, D/ 24 - D/30 = 9/60

Therefore D = 18
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Can someone explain why this reasoning is wrong:

I assumed for path 1, time = t

for path 2, time = t - 9/60

Using this method doesn't yield the same results, what is incorrect in my underlying assumption?

Thanks,
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omullick
Can someone explain why this reasoning is wrong:

I assumed for path 1, time = t

for path 2, time = t - 9/60

Using this method doesn't yield the same results, what is incorrect in my underlying assumption?

Thanks,

Path 1 : speed : 24km/hr ; time: t hrs
Path 2 : speed : 30km/hr ; time: t -9/60 hrs

therefore, Distance= 24*t = 30* (t-9/60)
t = 3/4 hrs
d = 24*t = 18
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I think this is a high-quality question and I agree with explanation. Its dev. test
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I have edited the question and the solution by adding more details to enhance its clarity. I hope it is now easier to understand.
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I think this is a high-quality question and I agree with explanation.
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I think this is a high-quality question and I agree with explanation.
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Bunuel
A soccer coach takes \(x\) hours to cycle from his house to his office. When riding at 24 km/h, he arrives 5 minutes late, but at 30 km/h, he arrives 4 minutes early. What is the distance between his house and the office?

A. 18 km
B. 24 km
C. 36 km
D. 40 km
E. 72 km


When his speed is 30 kmph, he reaches at say 12:00 noon (which is 4 mins early)
So normally he reaches at 12:04.
When his speed is 24 kmph, he reaches at 12:09 (5 mins late)

Hence the diff between time taken with speed 24 to speed 30 is 9 mins.

Ratio of speeds = 24:30
Ratio of time taken = 30:24 = 5:4
The diff of 1 on ratio scale is equal to 9 mins which means time taken at speed of 30 kmph is 4*9 = 36 mins
Distance travelled at speed 30 kmph in 36 mins = \(30 * \frac{36}{60} = 18\) km

Answer (A)
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My solution is a little different and more on logic.

so Speed and Time are inversely proportional if speed increases by 25% then time will decrease by 20%.

So speed changes from 24 to 30, an, an increase of 25%, and hence time decreases by 20%. Now, 20% of something is 9 mins, so the time taken when speed was 24kmph is 45 mins.

hence distance is 18km
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I like the solution - it’s helpful.
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I like the solution - it’s helpful.
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