How do you know that calculating 80% of the regular price gives you the same result as subtracting 20% from the regular price

For working out percentages, you can use decimals e.g 80% = 0.8
Times the original price with 0.8
Once you've done that, times the original price by 0.2 then take that number away from your original price.

Another way could be that 80% + 20%= 100% so you know it's the same for both ways. Sorry if this is confusing, I hope it helps

Letter a i would say