Question 493834
Let a be the number of adult tickets sold. Similarly, let c be the number of child tickets sold. Ultimately, we will have a system of equations since there are two unknowns.

We know the total sales were 707.70:
6.10c + 9.20a = 707.70 

We also know that a = 3c since there were 3 times as many adult tickets as children tickets.  This is all we need.

Substitute a=3c into the first equation.

6.10c + 9.20(3c) = 707.70

6.10c + 27.60c = 707.70

33.70c = 707.70

c = 21  <---- Answer

Extra:

Now let's say that they wanted to know how many adult tickets were sold.

We would plug in c =21 into  a =3c  to find that 63 adult tickets were sold. 

Hope this helped!