Sum of the digits of the positive integer n where n<99

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gmattesttaker2: Legendary Member; Posts: 641; Joined: Tue Feb 14, 2012 3:52 pmFollowed by: 8 members; Thanked: 11 times

by gmattesttaker2 » Sat Jul 07, 2012 11:54 pm

Hello,

Can you please help with this:

What is the sum of the digits of the positive integer n where n < 99?

1) n is divisible by the square of the prime number y.
2) y4 is a two-digit odd integer.

Answer given is C

1) If, n = 25 => 25/(5.5)
n = 49 => 49/(7.7)

Insufficient. However, I am not able to proceed after this. Can you please assist? Thanks a lot.

Best Regards,
Sri

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Source: Beat The GMAT — Data Sufficiency | Sat Jul 07, 2012 11:54 pm

eagleeye: Legendary Member; Posts: 520; Joined: Sat Apr 28, 2012 9:12 pmFollowed by: 49 members; Thanked: 339 times; GMAT Score:770

by eagleeye » Sun Jul 08, 2012 12:30 am

gmattesttaker2 wrote:Hello,

Can you please help with this:

What is the sum of the digits of the positive integer n where n < 99?

1) n is divisible by the square of the prime number y.
2) y4 is a two-digit odd integer.

Answer given is C

1) If, n = 25 => 25/(5.5)
n = 49 => 49/(7.7)

Insufficient. However, I am not able to proceed after this. Can you please assist? Thanks a lot.

Best Regards,
Sri

1: if prime number is 2, 2^2=4.
If n=16, n is divisible by 4, sum of digits =7
If n=20, n is divisible by 4, sum = 2
Hence Insufficient.

2: y^4 is a two-digit odd integer.
Doesn't tell us anything about n. Insufficient.

Together:
Smallest odd integer is 3. 3^4 = 81. Two digit and odd.
Next one is 5. 5^4 = 625. This is a 3 digit integer.
Therefore y=3.
Now n is divisible by y^2=3^2 = 9.
Now the cool thing is that if a 2 digit number, less than 99, is divisible by 9, the sum of its digits is always 9. (9, 18, 27......81 etc. all add upto 9).
Hence sum of digits of n is 9. Sufficient.

Hence C.

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Bill@VeritasPrep: GMAT Instructor; Posts: 1248; Joined: Thu Mar 29, 2012 2:57 pmFollowed by: 192 members; Location: Everywhere; Thanked: 503 times; GMAT Score:780

by Bill@VeritasPrep » Sun Jul 08, 2012 1:04 pm

A great example of a problem where knowing number properties is a great shortcut!

eagleeye wrote: Now the cool thing is that if a 2 digit number, less than 99, is divisible by 9, the sum of its digits is always 9. (9, 18, 27......81 etc. all add upto 9).

Also, this is true for all multiples of 9 regardless of number of digits. 279 = 2 + 7 + 9 = 18.

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gmattesttaker2: Legendary Member; Posts: 641; Joined: Tue Feb 14, 2012 3:52 pmFollowed by: 8 members; Thanked: 11 times

by gmattesttaker2 » Sun Jul 08, 2012 4:04 pm

eagleeye wrote:
gmattesttaker2 wrote:Hello,

Can you please help with this:

What is the sum of the digits of the positive integer n where n < 99?

1) n is divisible by the square of the prime number y.
2) y4 is a two-digit odd integer.

Answer given is C

1) If, n = 25 => 25/(5.5)
n = 49 => 49/(7.7)

Insufficient. However, I am not able to proceed after this. Can you please assist? Thanks a lot.

Best Regards,
Sri

1: if prime number is 2, 2^2=4.
If n=16, n is divisible by 4, sum of digits =7
If n=20, n is divisible by 4, sum = 2
Hence Insufficient.

2: y^4 is a two-digit odd integer.
Doesn't tell us anything about n. Insufficient.

Together:
Smallest odd integer is 3. 3^4 = 81. Two digit and odd.
Next one is 5. 5^4 = 625. This is a 3 digit integer.
Therefore y=3.
Now n is divisible by y^2=3^2 = 9.
Now the cool thing is that if a 2 digit number, less than 99, is divisible by 9, the sum of its digits is always 9. (9, 18, 27......81 etc. all add upto 9).
Hence sum of digits of n is 9. Sufficient.

Hence C.

Hello Eagleeye,

Thank you very much for your explanation. As always, it is very detailed and clear. Thanks a ton.
@Bill, thanks for your tip as well.

Best Regards,
Sri

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