Question 1077455
.
<pre>
Let x be the number of $10 tickets, and let y be the number of $30 tickets.

Then the number of $20 tickets is (x+129).

The system of two equations in two unknowns is

x + (x+129) + y = 3408.         (1)    (The total number of tickets)
10x + 20(x+129) + 30y = 64460   (2)   (The total sales)


Simplify and solve.
</pre>

<U>The lesson to learn from his solution</U>: 

<pre>
     This problem reduces to two unknowns.
     It is not about 3 unknowns.
</pre>