Question 994099
a adults
s students
c children
{{{a=-10+2s}}}
-
{{{system(a+s+c=278,6a+3.5s+2.5c=1300,a=2s-10)}}}


Multiply the members of the revenue equation by 2:
{{{system(a+s+c=278,12a+7s+5c=2600,a=2s-10)}}}
Three linear equations in three unknowns.  Solve.