Question 1088970
a, adults
c, children


{{{system(a+c=619,3a+c=1150)}}}


The convenient way to continue is Eliminate c, solving first for a.
{{{(3a+c)-(a+c)=1150-619}}}
{{{2a=531}}}
but this makes no sense.  Whole and EVEN number for a will be needed.  There cannot be any HALF of an adult.  Recheck your problem description.