A quadratic equation is in the form of x^2 - 2px + m = 0, where m is divisible

[BTGModeratorVI]

A quadratic equation is in the form of x^2 - 2px + m = 0, where m is divisible by 5 and is less than 120. One of the roots of this equation is 7. If p is a prime number and one of the roots of the equation, x^2 – 2px + n = 0 is 12, then what is the value of p+n–m?

A. 0
B. 6
C. 16
D. 26
E. 27

Answer: D
Source: E-gmat

[Brent@GMATPrepNow]

A quadratic equation is in the form of x^2 - 2px + m = 0, where m is divisible by 5 and is less than 120. One of the roots of this equation is 7. If p is a prime number and one of the roots of the equation, x^2 – 2px + n = 0 is 12, then what is the value of p+n–m?

A. 0
B. 6
C. 16
D. 26
E. 27

Answer: D
Source: E-gmat

GIVEN: x = 7 is one of the roots of the equation x² – 2px + m = 0
This means (x - 7) must be one of the factors of the expression on the left side of the equation.
That is, x² – 2px + m = 0, can be rewritten as (x - 7)(x +/- something) = 0 [notice that x = 7 is definitely a solution to the new equation]
Let's assign the variable k to the missing number (aka "something")
We can write: x² – 2px + m = (x - 7)(x - k)

GIVEN: m is divisible by 5 and is less than 120
We already know that: x² – 2px + m = (x - 7)(x - k)
If we expand the right side we get: x² – 2px + m = x² – kx - 7x + 7k
Now rewrite the right side as follows: x² – 2px + m = x² – (k + 7)x + 7k

We can see that 2p = k + 7
And we can see that m = 7k

In order for m to be divisible by 5, it must be the case that k is divisible by 5.
So, k COULD equal 5, 10, 15, 20, 25, etc
Let's test a few possible values of k

If k = 5, then 2p = 5 + 7 = 12
When we solve this, we get: p = 6
HOWEVER, we're told that p is PRIME
So, it cannot be the case that k = 5

If k = 10, then 2p = 10 + 7 = 17
When we solve this, we get: p = 8.5
HOWEVER, we're told that p is PRIME
So, it cannot be the case that k = 10

If k = 15, then 2p = 15 + 7 = 22
When we solve this, we get: p = 11
Aha! 11 is PRIME
So, it COULD be the case that k = 15. Let's confirm that this satisfies the other conditions in the question.

If k = 15, then we get: x² – 2px + m = (x - 7)(x - 15)
Expand and simplify the right side: x² – 2px + m = x² – 22x + 105
So, this meets the condition that says m is divisible by 5 and is less than 120

We now know that p = 11 and m = 105
All we need to do now is determine the value of n

GIVEN: x = 12 is one of the solutions of the equation x² – 2px + n = 0
Plug in x = 12 to get: 12² – 2p(12) + n = 0
Since we already know that p = 11, we can replace p with 11 to get: 12² – 2(11)(12) + n = 0
Simplify: 144 - 264 + n = 0
Simplify: -120 + n = 0
Solve: n = 120

What is the value of p + n – m?
p + n – m = 11 + 120 - 105
= 26

Answer: D

Cheers,
Brent

[Scott@TargetTestPrep]

A quadratic equation is in the form of x^2 - 2px + m = 0, where m is divisible by 5 and is less than 120. One of the roots of this equation is 7. If p is a prime number and one of the roots of the equation, x^2 – 2px + n = 0 is 12, then what is the value of p+n–m?

A. 0
B. 6
C. 16
D. 26
E. 27

Answer: D

Solution:

Since 7 is a root of x^2 - 2px + m = 0, we have:

7^2 - 2p(7) + m = 0

49 - 14p + m = 0

49 + m = 14p

7 + m/7 = 2p

Since m is a multiple of 5 and p is a prime, we see that m must be a multiple of 7 also. In other words, m is a multiple of 35, However, since 2p is even and 7 is odd, m/7 must be odd. In other words, m must be odd. Since m is less than 120, m can only be 35 x 1 = 35 or 35 x 3 = 105.

If m = 35, we have:

7 + 35/7 = 2p

12 = 2p

6 = p

However, since p is a prime, p can’t be 6, and hence m can’t be 35.

If m = 105, we have:

7 + 105/7 = 2p

22 = 2p

11 = p

So m must be 105 and p must be 11. Now, since 12 is a root of x^2 – 2px + n = 0 and we know p = 11, we have:

12^2 - 2(11)(12) + n = 0

144 - 264 + n = 0

-120 + n = 0

n = 120
Therefore, p + n - m = 11 + 120 - 105 = 26.

Answer: D