Child admission is $5.70 and adult admission is $9.50. twice as many adult tickets ad child tickets were sold. for a total of $543.40. how many child tickets were sold

Let the number of child tickets sold be x, and number of adult tickets be y, then
5.70x + 9.50y = 543.40 . . . (1)
y = 2x . . . (2)

Putting (2) into (1), gives
5.7x + 9.5(2x) = 543.4
5.7x + 19x = 543.4
24.7x = 543.4
x = 543.4/24.7 = 22

Therefore, 22 child tickets were sold.