GMAT Prep Probability

[naveenhv]

{-10, -6, -5, -4, -2.5, -1, 0, 2.5, 4, 6, 7, 10}

A number is to be selected at random from the set above. What is the probability that the number will be a solution to the equation (x-5)(x+10)(2X+5) = 0?

a) 1/12

b) 1/6

c) 1/4

d) 1/3

e) 1/2

[jnelson0612]

naveen, for your own learning I'm going to ask you to do a little work here. What does it mean when a "number will be a solution to the equation (x-5)(x+10)(2X+5) = 0"? It means that one of those terms will be equal to zero so that when all three are multiplied together the result is zero.

In that case, what are the values of x that will result in a product of zero?

Once you have answered that, it should help you determine the probability of choosing one of those numbers from the set above. Remember, probability is the number of desired outcomes/the number of total outcomes.

Take a shot at this and then please post what you get. Thanks!

[krishnan.anju1987]

The answer is 1/4 right?

[RonPurewal]

The answer is 1/4 right?

no.

there are 12 numbers in the set, only two of which (-10 and -2.5) are solutions to the equation, so you're looking at 2/12 = 1/6.

i guess you just assumed that all three solutions to the equation are actually present in the set. whoops! bad assumption.
unfortunately, you can't be lazy here; you have to go through the set and check whether the numbers are actually there.

[krishnan.anju1987]

oops, Thanks.. I in a hurry did not check the sign for 5. Thanks for clearing it up.. I have got to stop making such mistakes.

[RonPurewal]

oops, Thanks.. I in a hurry did not check the sign for 5. Thanks for clearing it up.. I have got to stop making such mistakes.

yep

also, make sure you don't just dismiss that as a "silly mistake". the purpose of the math component, on this exam, is not just to test math -- it's just as much (if not more) to test all these mundane matters of organization, diligence, and so on.