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GMAT1900
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Combinations and DS 22 Mar 2007, 14:50
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Hello All Here are two problems I was wondering if u guys could help me out with these



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Combinations and DS 22 Mar 2007, 16:05
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tlntd177
Hello All Here are two problems I was wondering if u guys could help me out with these
Show more


1) Prob. Stock exchange 1,2,3-letters code

(E) 18278

X - number of one-letter codes
XY - number of two-letter codes
XYZ - number of three-letter codes

we need to calculate X + XY + XYZ = ?

X = 26. it is only 26 variations of the one-letter codes

XY = 26*26. Please don't forget that the letters may be repeated. Ex. : AB, BA, AA, BB

XYZ = 26*26*26. same trick is here. AAA, BBB, AAB, BAA, ....

Answer: 26+26*26+26*26*26 = 18278.

[comments]
So here we get 1 trick about repeating letters. If u miss it u may think that XY = 26*25.

Another one u must calculate 26*26 very quickly. -) speed up. -)
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XxxyyY
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Combinations and DS 22 Mar 2007, 16:18
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1) 26
2) 26P2 + 26
3) 26P3 + 26 + (25*26)*3

=26+650+26+15,600+(25*26*3)+26= 18,278

-E-
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Combinations and DS Updated on: 22 Mar 2007, 22:10
Originally posted by kyatin on 22 Mar 2007, 18:17.
Last edited by kyatin on 22 Mar 2007, 22:10, edited 1 time in total.
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On 2nd I got B. Which anyway was evident from your attachement
But below is my method

Current time = (y/x) + (40-y)/1.25x = (1.25y + 40 -y )/ 1.25x ---eq1

New time if he travels all at speed x = (y/x) ---eq2

If you see we really need to know if we can calculate new time/current time.

So take division of eq2/eq1

[(y/x)] / [ (1.25y + 40 -y )/ 1.25x]

As you will see x cancels out to give

New time / current time = [(y)] / [ (1.25y + 40 -y )/ 1.25] ---eq3

As you see eq3 can be solved only when y is known. So answer is B.
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Combinations and DS 22 Mar 2007, 22:03
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kyatin
On 2nd I got B. Which anyway was evident from your diagram
But below is my method

Current time = (y/x) + (40-y)/1.25x = (1.25y + 40 -y )/ 1.25x ---eq1

New time if he travels all at speed x = (y/x) ---eq2

If you see we really need to know if we can calculate new time/current time.

So take division of eq2/eq1

[(y/x)] / [ (1.25y + 40 -y )/ 1.25x]

As you will see x cancels out to give

New time / current time = [(y)] / [ (1.25y + 40 -y )/ 1.25] ---eq3

As you see eq3 can be solved only when y is known. So answer is B.
Show more


agree. answer is B here.

we need to find eq1 = z% * eq2. --> 40/x - y/5x = z% * 40/x -->
1 - y/40 = z%*y.

the only thing we have to know is Y value. So (B) is our choice.



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This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
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Thank you for understanding, and happy exploring!
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