Let x be the cost of adult ticket and y be the cost of child ticket
2x + 4y = 210 ---------------------------------(1)
3x + 2y = 375 -----------------------------(2)
we can now solve equation (1) and (2) simultaneously
multiply through equation (1) by 3 and then through equation 2 by 2
6x + 12y = 630 ---------------------------(3)
6x + 4y = 750 ----------------------------(4)
equation (3) - equation (4)
8y = -120
y = -15
substitute y = -15 into equation (1) and then solve for x
2x + 4(-15) = 210
2x - 60 = 210
Add 60 to bot-side of the equation
2x = 270
Divide both-side of the equation by 2
x = 135
Hence the cost of one adult tickets cost $135