5 times 78 is 390, but what is the answer if both 5 and 78 are negative numbers? Here we will determine if the answer to negative 5 times negative 78 is positive 390 or negative 390.
There are many different ways to explain and determine if the answer to negative 5 times negative 78 is -390 or +390 using mathematical terms such as additive inverse and mathematical laws like the Distribute Law.
We will take a different approach - process of elimination: We will simply test if -390 is the correct answer using basic algebra.
We will test if -390 is correct by setting up the equation negative 5 times negative 78 equals negative 390, and then simplifying the equation to see if it is true.
(-5) (-78) = (-390)
Divide both sides of the equation by (-78) :
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| = |
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The numerator and denominator on the left side both have (-78) which even each other out and can be removed:
| (-5) = |
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The numerator and denominator on the right side both have a minus sign (-) which even each other out and can be removed:
| (-5) = |
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A numerator of 390 and denominator of 78 on the right side is the same as 390 divided by 78 which is 5. Now we end up with this untrue equation:
(-5) = (5)
-5 is NOT equal to 5, thus we have proven that negative 5 times negative 78 is not equal to -390. The answer must therefore be 390:
-5 x -78 = 390
The above applies to all math problems where you multiply two negative numbers. Multiplying two negative numbers such as negative 5 times negative 78 will always have a positive answer.
Negative Times Negative Calculator
Here you can multiply another two negative numbers together.
What is negative 5 times negative 79?
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