Stacey is selling tickets to the school play. the tickets are $7 for adults and $4 for children. she sells twice as many adult tickets as children's tickets and brings in a total of $378. how many of each kind of ticket did she sell?

Let the amount of adult tickets Stacey sold be 'A', and the amount of child tickets be 'C'.
We know the prices of each ticket, and hiw much she brought in, so we know that:
7A + 4C = 378
However, we also know that A = 2C, as she sold twice as much adult tickets as child tickets. Therefore 2A = 4C, so 2A - 4C = 0
When you subtract the equations;
(7A+4C) + (2A-4C) = 378 - 0
So 9A is 378, and A is 42, so Stacey sold 42 adult tickets.
We know that she sold half as many child tickets, so she had to sell 21 child tickets.

Always double-check your answers:
(7x42) + (4x21)
= 294 + 84
= 378, which was the original amount

So she sold 42 adult tickets and 21 child tickets

Hope this helped