Please help me work out the costs?

[Gilltthepill]

We are going on a short break with my niece.
Dh, me, ds and niece. Two bed apartment with a double bedroom, twin bedroom and sofabed. I can’t share with dh as I snore and he gets up to go to the loo. Dc is a late teen boy. So, I will share twin room with my niece (she knows about the snoring, we’ve shared a tent before), dc will have the double bed and dh sofa bed.

We are staying for 5 nights, niece staying for 3 nights. Total cost £645. How much should niece pay (she is an adult and works full time).

I'd say work out cost per night which is £129. Then divide that by 4 people and multiply by 3 nights. So it would be £96.75. Others might have a different way of working it out.

Great, sounds about right @Crinkle77 thank you.

I would divide by 5 Niece lays 1/5.

reason being your teen gets a double room to himself. Your DH a sofa bed to himself and your bribe has to share a twin with you (when DH won’t/ can’t share).

then divide that by 5 nights and she pay for 3 of those nights of 1/5 share

DS should pay the higher cost per night as he has his own room. DH has a room to himself but I assume that’s the living room so cost should be lower. Then you and niece should split the cost of a room.
So I would say around £75-£80. However, if without your niece you would have booked a 1 bed flat which would have been considerably cheaper, I would also take that into account when determining the price.