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# Two positive integers a and b are divisible by 5, which is their

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Intern
Joined: 13 Mar 2016
Posts: 1
Re: Two positive integers a and b are divisible by 5, which is their  [#permalink]

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19 Apr 2017, 09:06
I just have one doubt wrt statement 1. Where is that another 5 coming from?
Manager
Joined: 18 Jan 2018
Posts: 50
Re: Two positive integers a and b are divisible by 5, which is their  [#permalink]

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18 Feb 2018, 02:00
Tough one to interpret & crack correctly.
Senior Manager
Status: love the club...
Joined: 24 Mar 2015
Posts: 272
Re: Two positive integers a and b are divisible by 5, which is their  [#permalink]

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30 Mar 2018, 04:55
EgmatQuantExpert wrote:
Detailed Solution

Step-I: Given Info

We are given two positive integers a and b such that their highest common factor is 5. We are asked to find the values of a and b.

Step-II: Interpreting the Question Statement

Since, the highest common factor of a and b is 5, it implies that the GCD of (a, b) = 5. So, we can deduce that both a and b definitely have 5 as one of their prime factors. Let’s use the information given in the statements to see if we can find out the values of a and b.

Step-III: Statement-I

Statement-I tells us that the LCM (a, b) is the product of one of the integers of a and b with GCD (a, b) i.e. 5. Since, we know that the product of the numbers is equal to the product of their LCM & GCD, we can write
LCM * GCD = a * b

⇨ 5 * (a or b) * 5 = a * b
Simplifying this statement would give us the value of one of the number as 25. Since, we do not have any information about the value of the second number, statement-I is insufficient.

Step-IV: Statement-II

Statement-II tells us that the smaller of the two integers a and b is divisible by 4 factors and has the smallest odd prime number (i.e. 3) as its factor. So, the integer can be represented as:
x * y or x^3.
Since, we know that both the integers have 5 as one of its prime factors (since 5 is the GCD) so, we can represent the integer as product of two prime factors i.e. x * y = 3 * 5= 15.
Now, we know that one of the integers has the value of 15, but we are not given any information about the value of the second integer.

Hence, Statement-II is insufficient to answer the question.

Step-V: Combining Statements I & II

Statement- I tells us that one of the integers of a and b has the value as 25. Statement-II tells us that the other integer has the value as 15. This would mean that:
Either a= 25 and b =15 or
a = 15 and b =25.
Hence, we can’t say with certainty the specific values of a and b.

Thus, combining St-I & II is also not sufficient to answer the question.
Hence, the answer is Option E.

Key Takeaways

1. The prime factors of the GCD of a set of numbers would also be the prime factors of the numbers themselves.
2. Familiarize yourself with all the names by which the test makers can call the GCD and the LCM.

For example,
• GCD is also known as the HCF
• GCD can also be described as ‘the largest number which divides all the numbers of a set’
• LCM of a set of numbers can also be described as ‘the lowest number that has all the numbers of that set as it factors’

Harley1980 I hope you realized where did you miss

Regards
Harsh

Harsh
hi

since both integers have 5 as their prime factors, and statement 2 says that smaller integer has 3 as its prime factor, we can say one integer is 5 * 3 = 15

now, can you please tell me why to assume that the smaller integer doesn't have any other prime factors other than 3 and 5..?

is this because, 15 has exactly 4 factors, a condition met ?

Manager
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Posts: 135
Re: Two positive integers a and b are divisible by 5, which is their  [#permalink]

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14 Oct 2018, 23:48
I arrived at 15 and 25 and then was tempted to smash C omitting that I still didn't know which one is "a" and which one is "b"

GMATH Teacher
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Re: Two positive integers a and b are divisible by 5, which is their  [#permalink]

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15 Oct 2018, 05:06
Quote:
Two positive integers a and b are divisible by 5, which is their largest common factor. What is the value of a and b?

(1) The lowest POSITIVE number that has both integers a and b as its factors is the product of one of the integers and the greatest common divisor of the two integers.

(2) The smaller integer is divisible by EXACTLY 4 POSITIVE numbers and has the smallest odd prime number as ONE OF its factors.

$$\left. \matrix{ a,b\,\, \ge 1\,\,{\rm{ints}} \hfill \cr {\rm{GCD}}\left( {a,b} \right) = 5\,\, \hfill \cr} \right\}\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\left\{ \matrix{ \,a = 5M\,\,,\,\,M \ge 1\,\,{\mathop{\rm int}} \, \hfill \cr \,b = 5N\,\,,\,\,N \ge 1\,\,{\mathop{\rm int}} \, \hfill \cr} \right.\,\,\,\,\,\,M,N\,\,{\rm{relatively}}\,\,{\rm{prime}}$$

$${\rm{?}}\,\,\,{\rm{:}}\,\,\,{\rm{a,}}\,{\rm{b}}$$

Let´s go straight to (1+2), because it is easy to BIFURCATE (1) and (2) together (and this guarantees that each alone is also insufficient):

$$\left( {1 + 2} \right)\,\,\,\left\{ \matrix{ \,{\rm{Take}}\,\,\left( {a,b} \right) = \left( {5 \cdot 3,5 \cdot 5} \right)\,\,\,\,\left( * \right)\, \hfill \cr \,{\rm{Take}}\,\,\left( {a,b} \right) = \left( {5 \cdot 5,5 \cdot 3} \right)\,\,\,\,\left( {**} \right)\,\, \hfill \cr} \right.$$

$$\left( * \right)\,\,\,\left\{ \matrix{ LCM\left( {a,b} \right) = 3 \cdot {5^2} = \left( {5 \cdot 3} \right) \cdot 5 = a \cdot GCD\left( {a,b} \right)\,\,\,\,\,\, \Rightarrow \,\,\,\left( 1 \right)\,\,{\rm{satisfied}} \hfill \cr \,{\rm{5}} \cdot {\rm{3}}\,\,\,{\rm{has}}\,\,{\rm{exactly}}\,\,{\rm{4}}\,\,{\rm{positive}}\,\,{\rm{divisors}}\,\,{\rm{and}}\,\,{\rm{3}}\,\,{\rm{is}}\,\,{\rm{one}}\,\,{\rm{of}}\,\,{\rm{its}}\,\,{\rm{factors}}\,\,\,\,\,\, \Rightarrow \,\,\,\left( 2 \right)\,\,{\rm{satisfied}} \hfill \cr} \right.$$

$$\left( {**} \right)\,\,\,{\rm{analogous}}\,\,$$

This solution follows the notations and rationale taught in the GMATH method.

Regards,
Fabio.
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Re: Two positive integers a and b are divisible by 5, which is their &nbs [#permalink] 15 Oct 2018, 05:06

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