Question 424727
x children
y adults


{{{system(x+y=275,4x+7y=1174)}}}


Easiest seems try 4 times the people count.

{{{system(4x+4y=1100,4x+7y=1174)}}}

Subtract.
{{{3y=74}}}
{{{y=24&2/3}}}
This can not be.  Whole number values are necessary.


Recheck the problem description from your source.