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06 May 2015, 02:36
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D
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Difficulty:
95%
(hard)
Question Stats:
28% (02:36) correct
72%
(02:44)
wrong
based on 659
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In the correctly-worked multiplication problem above, each symbol represents a different nonzero digit. What is the value of C?
(1) D is prime.
(2) B is not prime.
Kudos for a correct solution.
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AlphameticPost3.jpg [ 3.66 KiB | Viewed 18645 times ]
11 May 2015, 04:43
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Bunuel
VERITAS PREP OFFICIAL SOLUTION:
We multiply two two-digit integers and get 1995. The good thing is that we know the result of the multiplication will be 1995. Usually, multiplication alphametics are harder since they involve multiple levels, but here the multiplication is actually a blessing. There are many many ways in which you can ADD two integers to give 1995 but there are only a few ways in which you can multiply two integers to give you 1995.
Let’s prime factorize 1995:
1995 = 3*5*7*19
We can probably count on our fingers the number of ways in which we can select AB and CD.
19 needs to be multiplied with one other factor to give us a two digit number since 5*3*7 = 105 (a three digit number) so AB and CD cannot be 19 and 105.
19*3 = 57, 5*7 = 35 – This is not possible since two of the four digits are same here – 5.
19*5 = 95, 3*7 = 21 – This is one option for AB and CD.
19*7 = 133 – Three digit number not possible.
Hence AB and CD can only take values out of 21 and 95.
As of now, C can be 2 or 9. We need to find whether the given statements give us a unique value of C.
Statement 1: D is prime
D is the units digit of CD. So D can be 1 or 5.
1 is not prime so CD cannot be 21. Hence, CD must be 95 and AB must be 21.
Hence, C must be 9.
This statement alone is sufficient.
Statement 2: B is not prime
If B is not prime then AB cannot be 95. Hence AB must be 21.
This means CD will be 95 and C will be 9.
This statement alone is sufficient.
Answer (D)
Note that the entire question was just about number properties – prime factors, prime numbers etc. Actually it required no iterative steps and no hit and trial. Rest assured that if it is a GMAT question, it will be reasoning based and will not require painful calculations.
06 May 2015, 18:34
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AB x CD = 1995... breaking 1995 into its prime factors - 3 x 5 x 7 x 19.. looking to find combinations of 2 digits from this 4 prime factors, will lead to only possibility of 21 x 95 i.e., other options are not possible like 35 x 57... 5 repeats here.. other 2 possibilities result in 3 digit numbers..
statement (1) D is prime.. meaning D=5 only possibility.. hence sufficient
statement (2) B is not prime.. meaning B=1 only possibility.. hence sufficient
Hence D.
statement (1) D is prime.. meaning D=5 only possibility.. hence sufficient
statement (2) B is not prime.. meaning B=1 only possibility.. hence sufficient
Hence D.
