Question 1092025
x, y, z - first second third

(Exact translation from written description):
{{{system(y=2x,z=y+4)}}}


Put x and z in terms of y and then avoid using them as variables.

{{{system(x=y/2,z=y+4)}}}



{{{y/2+y+(y+4)=39}}}-----one equation, one variable.

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{{{y/2+2y+4=39}}}

{{{y/2+2y=35}}}

{{{y+4y=70}}}

{{{5y=70}}}

{{{highlight(y=14)}}}--------You can use this length to evaluate the other two lengths.




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x=y/2,x=7;
z=y+4=14+4=18
check:  x+y+z=39?
7+14+18=21+18=39.