The integers m and p are such that 2 , and (m is not a factor of p. If r...
GMAT Data Sufficiency : (DS) Questions
The integers \(\mathrm{m}\) and \(\mathrm{p}\) are such that \(2 < \mathrm{m} < \mathrm{p}\), and \(\mathrm{m}\) is not a factor of \(\mathrm{p}\). If \(\mathrm{r}\) is the remainder when \(\mathrm{p}\) is divided by \(\mathrm{m}\), is \(\mathrm{r} > 1\)?
- The greatest common factor of \(\mathrm{m}\) and \(\mathrm{p}\) is \(2\).
- The least common multiple of \(\mathrm{m}\) and \(\mathrm{p}\) is \(30\).
Understanding the Question
We're asked whether \(\mathrm{r} > 1\), where r is the remainder when integer p is divided by integer m.
Given Information
- m and p are integers
- \(2 < \mathrm{m} < \mathrm{p}\)
- m is NOT a factor of p (so p isn't divisible by m)
- r is the remainder when p is divided by m
What We Need to Determine
Since m doesn't divide p evenly, we know there WILL be a remainder. By the division algorithm, when we divide p by m, we get: \(\mathrm{p} = \mathrm{mq} + \mathrm{r}\), where \(0 < \mathrm{r} < \mathrm{m}\).
The question asks: Is this remainder greater than 1? In other words, is r at least 2?
Key Insight
Since r must be between 1 and (m-1), and we're asking if r > 1, we're really asking: Can r be exactly 1, or must it be 2 or more?
Analyzing Statement 1
Statement 1: The greatest common factor of m and p is 2.
What Statement 1 Tells Us
If \(\mathrm{gcd}(\mathrm{m}, \mathrm{p}) = 2\), then both m and p are even numbers (they both have 2 as a factor).
The Strategic Insight
Here's the key realization: When we divide one even number by another even number, what happens to the remainder?
Think about it this way:
- p is even (let's say \(\mathrm{p} = 2\mathrm{k}\) for some integer k)
- m is even (let's say \(\mathrm{m} = 2\mathrm{j}\) for some integer j)
- When we divide p by m: \(\mathrm{p} = \mathrm{mq} + \mathrm{r}\)
This means: \(2\mathrm{k} = 2\mathrm{j} \times \mathrm{q} + \mathrm{r}\)
Rearranging: \(\mathrm{r} = 2\mathrm{k} - 2\mathrm{jq} = 2(\mathrm{k} - \mathrm{jq})\)
Since r equals 2 times some integer, r must be even!
Conclusion
The remainder r must be:
- Positive (since m doesn't divide p)
- Less than m
- Even (as we just proved)
The smallest positive even number is 2. Therefore, \(\mathrm{r} \geq 2\), which means \(\mathrm{r} > 1\).
[STOP - Statement 1 is SUFFICIENT!]
This eliminates choices B, C, and E.
Analyzing Statement 2
Now let's forget Statement 1 completely and analyze Statement 2 independently.
Statement 2: The least common multiple of m and p is 30.
What Statement 2 Provides
If \(\mathrm{lcm}(\mathrm{m}, \mathrm{p}) = 30\), then both m and p must be divisors of 30.
The divisors of 30 are: 1, 2, 3, 5, 6, 10, 15, 30
Given that \(2 < \mathrm{m} < \mathrm{p}\), the possible values for m are: 3, 5, 6, 10, 15
Testing Different Scenarios
Let's examine two specific cases to see if we always get the same answer:
Case 1: \(\mathrm{m} = 3\)
- For \(\mathrm{lcm}(3, \mathrm{p}) = 30\), we need p to be a multiple of 10
- Using the relationship: \(\mathrm{lcm}(3, \mathrm{p}) \times \mathrm{gcd}(3, \mathrm{p}) = 3\mathrm{p}\)
- We get: \(30 \times \mathrm{gcd}(3, \mathrm{p}) = 3\mathrm{p}\)
- This gives us \(\mathrm{p} = 10 \times \mathrm{gcd}(3, \mathrm{p})\)
- Since \(\mathrm{gcd}(3, 10) = 1\), we have \(\mathrm{p} = 10\)
- Dividing: \(10 = 3 \times 3 + 1\), so \(\mathrm{r} = 1\) ✗
Case 2: \(\mathrm{m} = 6\)
- For \(\mathrm{lcm}(6, \mathrm{p}) = 30\), using similar logic: \(30 \times \mathrm{gcd}(6, \mathrm{p}) = 6\mathrm{p}\)
- So \(\mathrm{p} = 5 \times \mathrm{gcd}(6, \mathrm{p})\)
- We need p > 6, and checking \(\mathrm{gcd}(6, 10) = 2\), we get \(\mathrm{p} = 10\)
- Dividing: \(10 = 6 \times 1 + 4\), so \(\mathrm{r} = 4\) ✓
Conclusion
We found:
- When \((\mathrm{m}, \mathrm{p}) = (3, 10)\): \(\mathrm{r} = 1\) (not greater than 1)
- When \((\mathrm{m}, \mathrm{p}) = (6, 10)\): \(\mathrm{r} = 4\) (greater than 1)
Since r can be either equal to 1 or greater than 1, we cannot determine whether \(\mathrm{r} > 1\).
Statement 2 is NOT sufficient.
This eliminates choices B and D.
The Answer: A
Statement 1 alone tells us that both m and p are even, which forces the remainder to be even (and therefore at least 2). Statement 2 alone allows for remainders of 1 or greater.
Answer Choice A: Statement 1 alone is sufficient, but Statement 2 alone is not sufficient.