SOLUTION: The average of three numbers is 12 and that of first two number is 9. Find the third number?

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Question 524950: The average of three numbers is 12 and that of first two number is 9. Find the third number?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let the numbers be a,b, and c
given:
(1) +%28a+%2B+b+%2B+c%29%2F3+=+12+
(2) +%28a+%2B+b%29%2F2+=+9+
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(2) +%28a+%2B+b%29%2F2+=+9+
(2) +a+%2B+b+=+18+
Substitute this back into (1)
(1) +%2818+%2B+c%29%2F3+=+12+
(1) +18+%2B+c+=+36+
(1) +c+=+36+-+18+
(1) +c+=+18+
The 3rd number is 18