Word problem Help please

Question

Suppose that there are two types of tickets to a show: advance and same-day. Advance tickets cost 

$15

 and same-day tickets cost 

$20

. For one performance, there were 

55

 tickets sold in all, and the total amount paid for them was 

$900

. How many tickets of each type were sold?

Answer 1

a = number of advance tickets at 15 each

s = number of same day tickets at 20 each

15a + 20s = 900

a + s = 55

a = 55 - s

15(55 - s) + 20s = 900

s = 15

a = 55 - s = 55 - 15 = 40

Test the answers.