I've just been given a water bill from my 'landlord'. It was $188.66. This is for 115 days. However I was only here for 29 of those days....

How much do I pay please?

should be $ 47.56 ( 188.66 divided by 115 = 1.64 x 29 = 47.56 )

Unreal that someone who can legally rent an apartment can't do 3rd grade math.

188.66/115 * 29 = $47.58.

Literally just need someone to do the maths for me.

Discuss it with your "landlord." We know nothing of your rental arrangement.

Do you share the apartment with anyone?