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From the condition, you have these 2 equations
x + y = 55 dollars (1)
20x + 25y = 1175 dollars (2)
whose meaning is clear without any explanations.
From equation (1), express x = 55-y and substitute it into equation (2). You will get
20*(55-y) + 25y = 1175
1100 - 20y + 25y = 1175
5y = 1175 - 1100 = 75
y = 75/5 = 15.
Then from equation (1), x = 55 - 15 = 40.
ANSWER. 40 advance and 15 same day tickets.
Check. 20*40 + 15*25 = 1175 dollars. ! Correct !