A small theater with 200 seats sells two kinds of tickets for a performance. Premium tickets sell for $20 each and Standard tickets sell for $15 each. At a sold-out performance, $3400 was collected in ticket sales. How many tickets of each type were sold?

p + s = 200......p = 200 - s
20p + 15s = 3400

20(200 - s) + 15s = 3400
4000 - 20s + 15s = 3400
-20s + 15s = 3400 - 4000
-5s = - 600
s = -600/-5
s = 120 <=== there were 120 standard tickets sold

p + s = 200
p + 120 = 200
p = 200 - 120
p = 80 <=== there were 80 premium tickets sold