Question 1149437
{{{system(d/n=7,10d=5n+390)}}}


{{{system(d=7n,2d=n+78)}}}


{{{14n=n+78}}}

{{{13n=78}}}

{{{highlight(n=6)}}}

{{{highlight(d=42)}}}


check:
{{{2*42=6+78}}}
{{{84=6+78}}}
{{{84=84}}}