In our local pub 0,4l pint of 4,5% beer cost 5.8 euros. However 0,33 l bottle of 4.7% beer cost 5 euros. I'm calculating how much cheaper pint is compare to bottle and how many pints I need to drink to get one for "free". I tried and failed
Cheaper how? In terms of beer, or in terms of alcohol?
Okay, we don't have the same standards for enjoying beer, but whatever.

The pints give you 4.5% of 0.4l = 18ml alcohol per 5.8 EUR. That's about 3.1034ml/EUR of alcohol. But I'm going to represent it as a fraction: 90/29ml/EUR.

The bottles give you 4.7% of 0.33l = 15.51ml alcohol per 5 EUR. That's exactly 3.102ml/EUR of alcohol.

So pints are only marginally cheaper, alcohol-content-wise: we're talking 0.00145ml/EUR alcohol difference. In terms of beer that's 0.00145/0.045 = 0.032ml/EUR beer, which is 0.032*5.8 = 0.19ml/pint. How good is your bartender? Because that's well within the margin of error for pouring a draught pint.

Anyway, assuming a perfect bartender, and assuming you don't spill any, to get a whole pint's worth of savings, you'd need to spend 18/0.00145 = 12,428.57 EUR. That's 12,428.57/5.8 = 2,142.86 pints.

Here in the UK, the gov recommends drinking no more than 14 units of alcohol per week. Each pint contains 18ml alcohol, which is 1.8 units. So you should drink no more than floor(14/1.8) = 7 pints each week. One per day -- luxury!

So it's going to take you 2,142.86/7 = 306.12 weeks to drink that many pints. That's almost 6 years.

I think we can assume that inflation is going to kick in well before that, totally ruining the calculation. So I suggest buying in bulk now. Buy 2,143, get 1 free!
The key is to drink 4 pints and then do your  mash. Wut waz cuestshun agean?
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I’m not good at ale-gebra