Seventy concert tickets were sold for $550. Each adult ticket cost $9 and each children's ticket cost $5. Find the number of adult tickets and the number of children's tickets sold.

Total tickets = 70

Total cost = $550

Adult's ticket = $9

Child's ticket = $5

Difference = 9 - 5 = $4

Assume all 70 are child's tickets

5 x 70 = 350

But the total cost was 550

550 - 350 = $200

The $200 must have come from Adult's tickets, which has a difference of $4.

200 : 4 = 50

So there are 50 adult's tickets.

70 - 50 = 20

And there are 20 child's tickets.