Simple inverse proportion maths question I couldn't answer. Can anyone tell me how this is calculated

[Eastie77]

A group of 6 friends want to go camping and they pack enough water to last for 4 days. Just as they are about to leave, they are joined by 2 more people. How long will their water last now?

So the answer is 3 days but I can't work out how to get that answer? I'm currently retaking Maths (obviously I need to!) and managed to answer all the other inverse proportion questions apart from this one...

Ok...think I worked it out. 1 person would have 24 days worth of water so 8 people would have 3 days worth.

I suppose there are a few ways to do it but one way is to work out the factor by which the number of campers has increased (ie 8/6 = 1.33333). Put another way, you have 1.33333 times more campers than you anticipated.

And then divide the number of days by that (ie 4/1.333333).

I'm sure someone will come along and do it much more elegantly!

They pack water for 6 people for 4 days = water for 24 person days. In fact 8 people go so the 24 person days of water will only last 24/8 days = 3 days.