GMAT Prep question

[anksm22]

Ans D

[Brent@GMATPrepNow]

If two of the four expressions x+y, x+5y, x-y, and 5x-y are chosen at random, what is the probability that their product will be of the form x² - (by)², where b is an integer?

A) 1/2
B) 1/3
C) 1/4
D) 1/5
E) 1/6

Important concept tested here
First recognize that x² - (by)² is a DIFFERENCE OF SQUARES.
Here are some examples of differences of squares:
x² - 25y²
4x² - 9y²
49m² - 100k²

In general, we can factor differences of squares as follows:
a² - b² = (a-b)(a+b)

So . . .
x² - 25y² = (x+5y)(x-5y)
4x² - 9y² = (2x+3y)(2x-3y)
49m² - 100k² = (7m+10k)(7m-10k)

--------------------------
From the 4 expressions (x+y, x+5y ,x-y and 5x-y), only one pair (x+y and x-y) will result in a difference of squares when multiplied.

So, the question now becomes:
If 2 expressions are randomly selected from the 4 expressions, what is the probability that x+y and x-y are both selected?

P(both selected) = [# of outcomes in which x+y and x-y are both selected]/[total # of outcomes]

As always, we'll begin with the denominator.

total # of outcomes
There are 4 expressions, and we must select 2 of them.
Since the order of the selected expressions does not matter, we can use combinations to answer this.
We can select 2 expressions from 4 expressions in 4C2 ways (= 6 ways)

If anyone is interested, we have a free video on calculating combinations (like 4C2) in your head: https://www.gmatprepnow.com/module/gmat-counting?id=789

# of outcomes in which x+y and x-y are both selected
There is only 1 way to select both x+y and x-y

So, P(both selected) = 1/6 = E

Cheers,
Brent

[Brent@GMATPrepNow]

If two of the four expressions x+y, x+5y, x-y, and 5x-y are chosen at random, what is the probability that their product will be of the form x² - (by)², where b is an integer?

A) 1/2
B) 1/3
C) 1/4
D) 1/5
E) 1/6

Alternate solution:

Once we recognize (from my last post) that we must find P(x+y and x-y are both selected), we can write:
P(x+y and x-y are both selected) = P(one of the two expressions is selected on 1st pick AND the other expression is selected on 2nd pick)
= P(one of the two expressions is selected on 1st pick) x P(the other expression is selected on 2nd pick)
= 2/4 x 1/3
= [spoiler]1/6[/spoiler]
= E

Cheers,
Brent

[GMATGuruNY]

If 2 of the expressions x-y, x+y, x+5y and 5x-y are chosen at random, what is the probability that product will be in the form of x^2 - (by)^2, where b is an integer?
a) 1/2
b) 1/3
c) 1/4
d) 1/5
e) 1/6

One approach is to FOIL every possible pair:

(x-y)(x+y) = x² - y²
(x-y)(x+5y) = x² + 4xy - 5y²
(x-y)(5x-y) = x² - 6xy + y²
(x+y)(x+5y) = x² + 6xy + 5y²
(x+y)(5x-y) = 5x² + 4xy - y²
(x+5y)(5x-y) = 5x² + 24xy - 5y²

Of the 6 results, only the first is in the required form:
x² - y² = x² - (1y)², where b=1.

Thus:
P(the product will be in the required form) = 1/6.

The correct answer is E.