It is the "in effect" that is the problem. From the 30 pounds, They paid 25 pounds to the hotel, 2 pounds to the porter and 3 pounds back to themselves.
So, from each original 10 pounds they put into the pool:
8 1/3 pounds each goes to the hotel 2/3 pound goes to the porter 1 pound goes to themselves
The £10 - £1 calculation disguises the fact that their original £10 was actually sliced up three ways, not two, and you've not reassembled all three pieces in yur final sum.
no subject
So, from each original 10 pounds they put into the pool:
8 1/3 pounds each goes to the hotel
2/3 pound goes to the porter
1 pound goes to themselves
The £10 - £1 calculation disguises the fact that their original £10 was actually sliced up three ways, not two, and you've not reassembled all three pieces in yur final sum.