QR word question.

[vDDmaniaC]

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I think, as of today, there are a lot of smart people who do very well on the QR section. So I just wanted to ask the problem which I have had and couldn't solve it. This is from the actual DAT 1990, but it is still a good source.

The question asks:
Anne's ski-boat can travel 20mph in calm water. On a river the boat could travel 45miles traveling with the current in the same time that it could travel 27 miles against the current. What is the rate of the current in miles per hour?

I am confusing with the setting up the coordination which corresponds to forward direction and the direction against the current.
In the forward direction: the gap 25mi between the boat speed w/current 45mi - boat speed it self 20mi.
In the opposite direction: there are 7mi different.
Then how do we have to deal with these two numbers in order to get the original current rate?

 

[doc3232]

Ok, Distance = Rate x time
So
45 miles = (20+current) x (time) We don't know the time
27 miles = (20-current) x (time) We still don't know the time
So
45 / (20 + current) = 27 / (20 - current) We don't need to know the time
Solve it out intermediate step: 900 - 45c = 540 + 27c where c=current

So c=5
there you go
You could also do guess and check but you will end up doing the same steps anyway.

 

[Andrew324]

Quick question, How do you do the 360/72 quickly? I guess it depends on the answers given at that point for me. But how would you do that one fast?

 

[vDDmaniaC]

thanks! doc3232. You are the man!

 

[klutzy1987]

Quick question, How do you do the 360/72 quickly? I guess it depends on the answers given at that point for me. But how would you do that one fast?

well 360/60 is exatly 6 which is very easy math. Therefore, your answer has to under 6 but not by too much. If 5 is a choice that is obviously the answer, unless the give you 4.9, 5, 5.1, 5.2 as your choices in which case it is time to do long division.

 

[Streetwolf]

360/72 = 180/36 = 90/18 = 45/9 = 5.

 

[doc3232]

Quick question, How do you do the 360/72 quickly? I guess it depends on the answers given at that point for me. But how would you do that one fast?

If you they are both divisible by 12 then you have a start.
StreetWolf's way is the best though, just start dividing by 2s.

 

[klutzy1987]

Nah that takes too long. Say you have bigger numbers. You need to look at your answer choices and see how close the numbers are. If they are not close at all then just an estimation will work just fine, you do not need to be exact usually.

 

[Streetwolf]

Well the #s weren't too big. I personally would go by 4s or higher. In fact if I didn't already know that 360/72 = 5, I'd start with 9 and reduce it to 40/8.

 

[bigasianpianist]

wow you guys are ridiculous! *in a good way*

 

[klutzy1987]

Well the #s weren't too big. I personally would go by 4s or higher. In fact if I didn't already know that 360/72 = 5, I'd start with 9 and reduce it to 40/8.

LOL that works just fine too. You probably see the factors right away. Im sure alot of people dont see it like that. Personally im with you, if you can see it then do it like that, looking for the greatest common factor.