The school play sold 410 tickets. The tickets were $6 at the door and president for $4. A total amount of $2000 was made from ticket sales. How many tickets were sold at the door?

It should read "$6 at the door and presold for $4"

Cannot determine from the info given. There needs to be more information. Think about it, there's no way to determine the number of tickets sold at the door vs. pre sold

Let d denote the number of tickets sold at the door, and p the number of pre-sold tix. Then the problem furnishes you with two linear equations:

d + p = 410
6d + 4p = 2,000

Solve for d and p either by substitution or by elimination.

Yup, that's the way to do it.

Of course, any other variable used is acceptable :)