We want to solve the following system of equations
and
where x is the number of adult tickets sold and y is the number of child tickets sold.
From the first equation, we can deduce that
If we replace this value on the second one, we get
If we operate on the left hand side, we get
Then, if we subtract 14 on both sides, we get
Finally, if we divide both sides by 3, we get
If we replace this value in the original equation, we have that
So 8 adult tickes and 7 child tickets were sold in total.