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a = b-6 ...(i)
a+4 = 1/2(b+4) ...(ii)
substitute equation (i) to (ii)
(b-6)+4 = 1/2(b+4)
b-2 = 1/2(b+4)
2(b-2) = b+4
2b-4 = b+4
unify the components with the same variable
2b-b = 4+4
b = 8, this is amy's brother age. substitute the number to equation (i)
a = 8-6
a = 2
amy's 2 years old and the brother is 8 years old.