Jim, this is going to be a system of equations.
Let's let c = the number of children and a = the number of adults.
Then, we know that a + c = 2000, right?
Since each children's ticket is $2, then the total made on children's tickets is $2c (If they sold 2 children's tickets, then made $4 on children's tickets, because $2 x 2 = $4).
For the same reason, the total made on adult tickets is $4a.
We know that 2c + 4a = 6000.
Let's go back to the first equation and "solve" for either a or c. I'll do a.
a = 2000 - c. Make sense?
We'll now take this "value" for a and plug it into the 2nd equation for a.
2c + (2000 - c) = 6000
2c + 2000 - c = 6000
I bet you can solve for c from here! Use this value of c, plug it into the first equation, and get a.
Hope this helps!