harsh.champ wrote:becnil wrote:It may be easier by way of Relative Speed. They have (51-11) 40 floors between them and their relative speed is (63+57) 120 floors per minute.
So, they will cross after 40/120 minutes, i.e. 20 seconds. In 20 sec, Jim rides down 63/3=21 floors, meaning they will cross at the (51-21) 30th floor.
Hey becnil,
You posted the same solution as mine above.
I was doubtful whether such a question can be asked on GMAT because they are not testing your physics skills in the exams.
Or is relative velocity a very common concept??
Coming from an engg background I don't know how a layman thinks??
Can someone throw some light in this regard !!
We're not talking relative velocity using relativity theory - it's simply a combined rate problem.
Here's the general rule: when two objects are travelling in opposite directions (i.e. directly toward each other or directly away from each other), you ADD their rates to get the combined rate.
In this question, one person is moving up and the other down, so the rule applies and we simply apply the distance/rate/time formula:
Total distance = total rate * total time
40 = 120 * t
1/3 = t
We can now use either person to answer the question.
If we choose Sam, in 1/3 of a minute he travels 1/3 * 57 = 19 floors. 11th + 19 = 30th floor.
If we choose Jim, in 1/3 of a minute he travels 1/3 * 63 = 21 floors. 51st - 21 = 30th floor.
* * *
We can also solve this question with weighted averages.
The combined rate is 120 fpm.
Sam covers 57/120 of the distance and Jim covers 63/120 of the distance.
If we solve with Sam, we get:
(57/120) * 40 floors = 57/3 = 19 floors. 11th + 19 = 30th floor.
If we solve with Jim, we get:
(63/120) * 40 floors = 63/3 = 21 floors. 51st - 21 = 30th floor.
