Problem 1 · 2016 Math Kangaroo
Medium
Arithmetic & Operations
total-then-divide
The arithmetic mean of four numbers is 9. What is the fourth number if the three other numbers are 5, 9 and 12?
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Answer: D — 10
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Hint 1 of 2
The mean times the count gives the total of all four numbers.
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Hint 2 of 2
Find the total, then subtract the three you know.
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Approach: mean as a total
- Four numbers with mean 9 add up to 4×9 = 36.
- The three given add to 5+9+12 = 26.
- The fourth number is 36 − 26 = 10.
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