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Let x = the hotel charge before tax in the first city;
y = that of the second city.
Then from the condition you have these two equations in two unknowns
x - y = 500 (1)
0.08x + 0.04y = 640. (2)
To solve the system of equations, use the Elimination method.
For it, from equation (1) express x = 500 + y and substitute it into the second equation. You will get
0.08*(500+y) + 0.04y = 640.
Simplify and solve for y
40 + 0.08y + 0.04y = 640
0.12y = 640 - 40 = 600
y = 600/0.12 = 5000.
ANSWER. In the second city hotel charged $5000; in the second city $500 dollars more, or 5500 dollars.
CHECK. 0.08*5500 + 0.04*5000 = 640 dollars. ! Correct !