Question 1096899
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Combined Steve, Matthew and Anthony's age sum to seventy-one. In three years, Anthony will be twice as old as Matthew. When Matthew is half Anthony's age, Steve will be five. I tried to work it out and put ...
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s, Steve
m, Matthew
a, Anthony
x, unknown amount of time passage in years


If you write a system of equations literally following the description, this can be that system, four equations in four variables.
{{{system(s+m+a=71,a+3=2(m+3),m+x=(a+x)/2,s+x=5)}}}

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{{{a+3=2m+6}}}
{{{a-2m=3}}}
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{{{2m+2x=a+x}}}
{{{2m+x=a}}}
{{{2m-a=-x}}}
{{{a-2m=x}}}
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{{{s+x=5}}}
{{{x=5-s}}}


A somewhat simplified system:
{{{system(s+m+a=71,a-2m=3,a-2m=x,x=5-s)}}}
which seems to indicate that {{{highlight_green(x=3)}}}, according to second and third equations. 


... if no mistakes so far, everything else should fall into place.



Notice this part of the description:
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In three years, Anthony will be twice as old as Matthew. When Matthew is half Anthony's age, Steve will be five.
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You may not need four equations in four unknowns - just three...