SajjadAhmad wrote:

Source: McGraw Hill GMAT

If xy = 10, what is the value of x + y?

1. x and y are positive numbers.

2. 1/x + 1/y = 3

Target question: What is the value of x + y? Given: xy = 10 Statement 1: x and y are positive numbers. This statement doesn't feel sufficient, so I'll TEST some values.

There are several values of x and y that satisfy statement 1 and the given information. Here are two:

Case a: x = 1 and y = 10, in which case

x + y = 1 + 10 = 11Case b: x = 2 and y = 5, in which case

x + y = 2 + 5 = 7Since we cannot answer the

target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: 1/x + 1/y = 3 Let's combine these fractions. To do so, we'll need a common denominator.

A common denominator of x and y is xy.

So, 1/x = y/xy and 1/y = x/xy

We can now take: 1/x + 1/y = 3

And rewrite it as: y/xy + x/xy = 3

Combine the fractions: (y + x)/xy = 3

Use the given information to get: (y + x)/

10 = 3

Multiply both sides by 10 to get:

y + x = 30PERFECT!

Since we can answer the

target question with certainty, statement 2 is SUFFICIENT

Answer:

Cheers,

Brent

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Brent Hanneson – Founder of gmatprepnow.com