# Lesson: QC Strategy - Plugging in Numbers

## Comment on QC Strategy - Plugging in Numbers

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### Is there any time when

Is there any time when plugging in numbers is better than matching operations? Because it seems as if plugging in numbers is more cumbersome and is rather a way to check your answer when you have first used the faster/more efficient matching operations

### Sometimes the Plugging in

Sometimes the Plugging in Numbers is the approach when you can't see how to apply the other approaches.
That said, if there are several complicated operations involved, sometimes it's faster to plug in some "easy values" (e.g., 0, 1, -1, etc) to see if you can reach a solution faster.

### Just wondering... Is it a

Just wondering... Is it a more foolproof and robust approach to plug in two integers with the same absolute value but with opposite signs (e.g., +1 and -1) to check for symmetry? I was thinking of always using +1 and -1. If they give me different results, then I know the answer is D. If they give me the same results, then try 0, just to be sure. Would you say this is a dominant strategy for this type of problem?

### That's a good strategy.

That's a good strategy.

Of course, if you get the same outcome with 1, -1 and 0, try 1/2, -1/2, 10 and -10, to see if you get a different outcome.

As mentioned in the video, plugging in numbers has its limitations since you will never by 100% certain of your answer UNLESS you're lucky enough to get two different outcomes (which means the correct answer is D).

### Thank you sir

Thank you sir

"Without two contradictory results, you cannot be certain of the correct answer."

Is this a grammar or logic mistake here? Don't you mean "with" two contradictory results? With 2 contradictory results (say you get A for one result and B for another result...these are contradictory), then you don't know what the answer is. So you should be saying "with" instead of "without."

### Let's say we have the

Let's say we have the following question:

x > 0
QUANTITY A: x
QUANTITY B: x²

Now let's test two different values of x.

case i: If x = 2, we get:
QUANTITY A: 2
QUANTITY B: 4
In this case, Quantity B is greater

case ii: If x = 0.5, we get:
QUANTITY A: 0.5
QUANTITY B: 0.25
In this case, Quantity A is greater

Now that we have two contradictory results (in one case, Quantity B is greater, and in the other case, Quantity A is greater), we can be certain the correct answer is D.
In other words, with two contradictory results, we can be certain of the correct answer.
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Now let's say we have the same question...
x > 0
QUANTITY A: x
QUANTITY B: x²
...but we test different values.

case i: If x = 2, we get:
QUANTITY A: 2
QUANTITY B: 4
In this case, Quantity B is greater

case ii: If x = 3, we get:
QUANTITY A: 3
QUANTITY B: 9
In this case, Quantity B is greater

Since we don't have two contradictory results, we can't be certain whether the correct answer is B or D.
In other words, without two contradictory results, we cannot be certain of the correct answer.

Does that help?

Cheers,
Brent