OK!
So the admission is $7.50 and it's 20% off so you need to find 20% of 7.50 by dividing 7.50 by 100 the multiplying the number you get by 20 to get the amount you need to subtract from 7.50 to get the discount. You can calculate tax the same way, buy taking the 7.50 / 100 and then multiplying it by 5.75. that will get you your tax and all you need to do is add it to the total!
Hope I helped!