Doesn't need to be per 100ml of course, could be any percentage of whatever unit you want, but the question remains the same.

Basically, there's a deal going on at a shop that I shop at. You can get 3x1.25l bottles of juice for 3, or I can forgo the deal and get 1x2l bottle for 1.45

Obviously, the 1x2l bottle is the better deal but I don't always know this for sure. I want to make a spreadsheet where I input the volume of the thing, the quantity of the thing, the total volume I'd get, and the total price I pay, and then it gives me price per 100ml.

In my spreadsheet, I've got

1 litre is equal to 1000 millilitres 

Volume-for-one-item which we'll call A = 1.25l
Quantity-of-item which we'll call B = 3
Total-Quantity which is just AB = 3.75

All super easy stuff

100ml is just 0.1 so I'll just use that because it's easy and can x 100 later if I want.

I need to multiply AB by some number and that is equal to 0.1. So If I divide AB by 0.1 then I get that special number which in this case is 37.5

So now, I can divide the total price by that number to get the price per 100ml and if I do that I get... 0.08?

Is that right? Am I doing it right?

Can anyone please suggest a useful app/website or hack for working out cost of perfume per mls? My mum brain has forgotten how to math 🥴