For the 20% off, we can make the following rule of three:
7.50 --------------> 100%
x ------------------> 20%
From here, we clear the value of x.
We have then:

Then, subtracting both values we have:

For taxes we have:
6 ---------------> 100%
x'---------------> 5.75%
Clearing the value of x' we have:

Then, adding both values we have:

Answer:
7.50 admission; 20% off; 5.75% tax is:
6.345