One thing to consider is that some people may not fully understand how a percentage works for this.
Since its Christmas time, and we will all be checking out those after Christmas sales lets look at how percentages apply to sales!
All right its the first day after Christmas, and in one particular store everything is 75% off.
To figure out the price of a $20 item we would multiply 20 x 75%. Ok thats fine, but what if you dont have a calculator? Well we have to remember that a percentage is always based on 100, as in 100% is full price, for one item. If you were set that equation up you could just say 20 x 1, since we're paying full price. Now if you think about that all we really did was move the decimal up two spaces, IE 20 x 1.00. With that in mind if we were to take our sale percentage, 75%, and move the decimal up two spaces we get .75, or 0.75 for Construct. We can then take our original price, 20, and multiply that times 0.75, and get 15. So we can now say 75% off of $20 is $5. If you wanted to check that in you head real quick you could say half of half. Half of 100 is 50, and half of 50 is 25, and 100 minus 25 is....75... sooooo half of 20 is 10, and half of 10 is 5. Simple ehh?
Now that we know that all we have to do is change the decimal, we can now easily figure out what the price will be on the second day when the sale price goes to 80%
20 x 0.8 =?
Anyway if you wanted to set that up in a lerp we would say lerp(0, 20,0.8).
Zero for a since thats the lowest price we can get in a sale. 20 for b, since thats the original price, and 0.6 for our current percentage. That gives us what we want to take off of our price.
But if you wanted to get the exact price we would change that to lerp(20,0,0.8)
Edit: fixed the top part...derr, Im a dumbass sometimes