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If a and b are integers, and 3^5*a=5^3*b, which of the following must

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If a and b are integers, and 3^5*a=5^3*b, which of the following must  [#permalink]

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New post 12 Apr 2017, 00:11
1
5
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A
B
C
D
E

Difficulty:

  65% (hard)

Question Stats:

58% (02:12) correct 42% (01:57) wrong based on 155 sessions

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Re: If a and b are integers, and 3^5*a=5^3*b, which of the following must  [#permalink]

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New post 12 Apr 2017, 03:37
Option C

: 3^5*a = 5^3*b
i.e., a = 5^3*b/3^5 & b = 3^5*a/5^3. Given a & b are integers.

b/27 = 3^2*a/5^3 = Integer always. For rest of the options, we need to ascertain the value of a & b.
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Re: If a and b are integers, and 3^5*a=5^3*b, which of the following must  [#permalink]

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New post 12 Apr 2017, 21:48
As we have no clue about the values of a and b. And we just know that a and b are integers, options A,B,D, and E cannot be determined.

Only option C is possible here because:

3^5*a= 5^3*b
a= 5^3*b/3^5

The right hand side must be an integer as a is an integer. Thus b carries at least 3^5.

Bunuel when the OA is released, please let me know if this method is correct.
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Re: If a and b are integers, and 3^5*a=5^3*b, which of the following must  [#permalink]

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New post 19 Apr 2017, 14:58
Bunuel wrote:
If a and b are integers, and 3^5*a=5^3*b, which of the following must be true?

A) b/(125) is an integer
B) a/(125*3^5) is an integer
C) b/(27) is an integer
D) a/(3) is an integer
E) a/(250) is an integer


We can simplify the given equation and we have:

3^5*a=5^3*b

(3^5)/(5^3) = b/a

Since (3^5)/(5^3) can’t be reduced further, we see that b is a multiple of 3^5 and a is a multiple of 5^3.

Therefore, of our answer choices, the only one that MUST be true is b/27 is an integer.

Answer: C
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Re: If a and b are integers, and 3^5*a=5^3*b, which of the following must  [#permalink]

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New post 11 May 2017, 11:51
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How are we even simplifying the main equation given in the text? How to bring power down and put it on one side of the equation?
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Re: If a and b are integers, and 3^5*a=5^3*b, which of the following must  [#permalink]

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New post 14 May 2017, 21:14
TheMastermind wrote:
How are we even simplifying the main equation given in the text? How to bring power down and put it on one side of the equation?


I don't the question is quoted properly. What I understood after first reading is

\(3^[5*a]=5^[3*b],\)

But the question says:

\(3^5*a = 5^3*b\)

Since we know that there are no primes common in \(3^5\)and \(5^3\)

a must contain \(5^3\) and b must contain \(3^5\) for inequality to hold true.

Thus choice C..
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Re: If a and b are integers, and 3^5*a=5^3*b, which of the following must  [#permalink]

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New post 31 Mar 2020, 00:13
1
TheMastermind wrote:
How are we even simplifying the main equation given in the text? How to bring power down and put it on one side of the equation?


Hello Mastermind,

Since all answer options are in terms of a and b, we try to express a in terms of b or vice versa. This approach is fairly common when an equation is given where there are two variables and the co-efficients cannot be simplified.

You see that the co-efficient of a = \(3^5\) i.e. 243 and the co-efficient of b = \(5^3\) i.e. 125. Clearly, 243 and 125 do not divide each other. Hence, we should think of expressing a in terms of b or vice versa.

Expressing a in terms of b, we have a = \(\frac{125b }{ 243}\). Since 125 is not divisible by 243, b HAS to be divisible by 243. If b is divisible by 243, it will also be divisible by ALL FACTORS of 243. Therefore, \((\frac{b}{27}\)) is definitely an integer since 27 is a factor of 243.

Answer option C is the correct answer.

Hope that answers your question!
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Re: If a and b are integers, and 3^5*a=5^3*b, which of the following must   [#permalink] 31 Mar 2020, 00:13

If a and b are integers, and 3^5*a=5^3*b, which of the following must

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