Answer:
let
c = the number of covered pavilion tickets sold
l = the number of lawn seats tickets sold
one evening 1400 concert tickets were sold
c + l = 1,400
total receipts were $30,000
30c + 15l = 30,000
by solving the system of equations
c + l = 1,400
30c + 15l = 30,000
we find
c = 400 covered pavilion seats
l = 1,200 lawn seats
by plugging the correct numbers into each other