Question 693525
m is for Mark
s is for his sister
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given equations
m = s + 11
m+8 = 2(s+8) ==> adding 8 to both m and s to account for 8 years later
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substitute m of the first equation into the second
s+11 + 8 = 2(s+8) ==> distribute the 2
s+11 + 8 = 2s + 16 ==> combine like terms
s + 19 = 2s + 16 ==> group s variables on one side
s - 2s = 16 - 19 ==> combine like terms
-s = -3  ==>solve for s
s = 3
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Mark is 3+11 = {{{highlight_green(14)}}}
His sister is {{{highlight_green(3)}}}
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