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the average of 3 numbers is 12. If one of the numbers is 8 what are the sum of the other 2 numbers

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2 Answers

Let the variables X & Y represent the 2 missing numbers.
 
So, (8 + X + Y)/3 = 12
 
multiply both sides of the equation by 3 to get 8 + X + Y = 36
 
subtract 8 from both sides of the equation to get X + Y = 28
 
So, the sum of the 2 missing numbers is 28!
 
To check 8 + 28 = 36  and 36/3 = 12
 
Good luck!
 
Steve
Since an average is a sum of numbers divided by the quantity of numbers, I would suggest working this backwards.  
 
Average = (sum of the numbers)/quantity
 
So, in this case, we know that the quantity of numbers (how many there are) is 3, and we know that the average is 12.
 
12 = sum/3
 
To solve this equation, we can multiply both sides by 3.
 
12 (*3) = sum/3 (*3)      which leaves us with...
 
36 = sum of the three numbers
 
But we know that one of the numbers is 8, so 36 - 8 = 28.  The sum of the other 2 numbers is 28.