Question 1092658
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David's dad is twice his age and five more, the sum of their age is 71. 
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x, David
y, Dad


{{{system(y=2x+5,x+y=71)}}}

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{{{x+2x+5=71}}} through simply substituting;

{{{3x+5=71}}}

{{{3x=71-5}}}

{{{3x=66}}}

{{{highlight(x=22)}}}--------David's age

{{{y=2*22+5}}}

{{{highlight(y=49)}}}-------Dad's age