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This is when you make up numbers to insert into a question.
This will usually occur when there are variables
in the question and variables in the answer choices as well
– questions so abstract that you shake your head when reading them!
Pick smart numbers. The
numbers you choose for Plug In/Testing Numbers should fit the question's parameters.
For example, if the question asks for an integer, you should insert
integers. Usually try plugging in a few different numbers (positive,
negative, zero, etc.). When testing numbers for Plug-In, always try picking 0, -2, +2, fractions, etc.. or any numbers that may be particular to the question. These will help check your answers. Pick a variety of numbers to make sure that you are exploring all possible reasonable scenarios.
Plug in (Testing Numbers) Pros and Cons
- Used very effectively in Data Sufficiency questions to test if the statements are sufficient. Use Testing Numbers constantly on Data Sufficiency as you use Backsolve constantly on multiple choice questions.
- On some geometry questions it can be effective to pick angles and lengths to see if they work as a test.
- Use in questions with small and finite pools of possible answers:
How many primes between 11 and 30 satisfy this statement?
Start substituting prime numbers to see if it works. This works wonderfully on small sets of numbers because you can test a few numbers without spending too much time.
- Use in questions where you have to prove true or false
Is ab > 0?
For a question like this, it is logical to test the conditions using a variety of numbers.
- Many people find it easier to do calculations with numbers rather than variables. So substituting numbers for variables can speed up your math. Plugging in numbers and "trying them out" can speed up the process.
Very often when you see a question, it will boil down to: should I do the long algebra or just Plug In / Backsolve?
This means that many GMAT questions have two distinct solution strategies. 800score math explanations will often feature two approaches: algebra and/or Backsolving/Plug-In.
- Sometimes algebra is quicker (especially when you are good at seeing shortcuts).
y + 1 < 1/2
If n is an even integer, which of the following must be an odd integer?
a) 3n - 2
b) 3(n + 1)
c) n - 2
d) n/3
e) n/2
Solution
(B) This
question has both variables in the question
and in the answer, so we need to plug in our
own numbers. Make n equal to 2. If n is 2, then 3(n + 1) = 9. Since our target is
an odd integer, this answer choice works. Try
a few more numbers to double check. For example,
2 may work with choice (e) to make an odd number
(1), but it will not work with any other even
numbers.
Note: When
plugging in, if you get a result of two or more
answer choices being correct, you must plug
in again. In the second case you should probably
plug in a number you know will disprove a certain
answer choice, or a number that is totally different
from the initial number plugged in (larger than
the original number, even if the original was
odd, etc.)
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