Mr. Alfred's favorite number (A)
Mr. Coble's favorite number (C)
A + C = 28 (I)
A - 2C = 10 (II)
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Solving the equations system by substitution
Rearranging (I)
A + C = 28
A = 28 - C
Replacing (I) in (II)
(28 - C) - 2C = 10
28 -3C = 10
3C= 28 - 10
C= 18 /3
C= 6
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Answer
Mr. Coble's favorite number is 6
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Verifying
A = 28 - C
A= 22 (Mr. Alfred's favorite number )
A + C = 28 (I)
22 + 6 = 28
A - 2C = 10 (II)
22 - 6*2 = 10
22- 12= 10
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