let 'x' be the number of tickets for adults (@40)
'y' be the number of tickets for children (@20)
since the total number of tickets is 2,000 then you'll have the equation as
x + y = 2000 ---equation 1
for the total amount:
40x + 20y = 64,000 ---equation 2
from equation 1
x + y = 2,000
x = 2,000 - y ------equation 1'
substitute equation 1' to equation 2
40x + 20y = 64,000
40(2,000 - y) + 20y = 64,000
80,000 - 40y + 20y = 64,000
80,000 - 64,000 = 40y - 20y
16,000 = 20y
20y = 16,000
y = 800
substitute y=800 to equation 1'
x = 2,000 - y
x = 2,000 - 800
x = 1,200
