Ok, so it's not A level maths. But it's always very, very confusing.
Always, at least one size of pack is on special offer, usually a multibuy special offer. The little labels which tell you the cost per nappy apply only to a single buy, so don't help you here. I was on Ocado a minute ago, looking for size 4+ nappies, and had to work out whether 3 lots of 28 for £15 was better than 30 for £4.32, or 74 for £9.85, or two lots of 48 for £16, or two lots of 70 for £22....
This is not obvious. Not easy. I have a maths degree and am not afraid to use it, but I resent the cynical and obvious marketing strategy which is to bamboozle parents as much as possible.
(Disclaimer: this is not a big AIBU, just a minor irritation like having your digestive disintegrate in your coffee. Just wanted to share the moan. Thank you.)