Will Dean's Ideas Factory: Billr app means an end to asking 'Why am I paying for your steak?'

The Billr app works for up to 16 diners

Somehow, it's never easy. Even if there's an even number of you. Even if everyone has the same meal. And the same drinks. Even the tip can make things complicated.

Yep, dining out in large groups is great fun until the dreaded moment when it's time to work out who owes what. Especially if one person isn't drinking alcohol while Dave at the end of the table is on his tenth Jägerbomb (or glass of Chablis, depending on the kind of places you frequent).

Ditto someone ordering the scallop starter and the fillet steak while someone else has a garden salad... And then there's someone who's paying for two people on a card and the other person paying cash for themselves.

The whole thing is a minefield – especially when it's being navigated by an innumerate armed with only a ballpoint pen and four beers in their stomach.

Well done, then, to developer Stephen Poletto and the other two members of the team behind Billr. The app – which works for up to 16 people and costs 69p – will work out who owes what, and how much of the tip they should contribute. Et voilà!

The idea isn't completely new; there are other iPhone apps available – such as the rather maladroit-sounding Tip & Bill Split App, or Android's well-regarded version called Split the Bill. What makes Billr great – despite the trite mssng-vwl-style name – is its retro look from industrial-design student Ivy Hu, which manages to make wrangling over who owes precisely what for their prawn wonton soup look almost cool. billr.me

Transforming the subway into a tourist destination

The problem with advertising on a transport system is that it's hard to make people stop and take notice. If they're not half-awake and concentrating on getting to the office/home, they're plugged into headphones or stuck in a book.

You imagine this might not be the case for commuters trudging through Montgomery Street station in downtown San Francisco over the next few weeks. A corridor in the Bart (Bay Area Rapid Transit) station has been transformed by the creative agency Struck to create the impression that passengers are walking through the Delicate Arch, one of the rock formations found at Utah's Arches National Park.

The design is, of course, on behalf of the Utah tourist board, and a bid to encourage San Fran residents to take a trip east to Utah. The installation is so brilliant, though, that it's already being disseminated around the web and, of course, into the mills of the Ideas Factory – meaning that it's not just Montgomery Street commuters who might reconsider a trip to the Beehive State. Good work.

See more: ind.pn/struckutah

The book exchange gets a home

Book swapping – leaving old books around for someone to exchange with one of theirs – isn't a new concept. But the idea of entire shops dedicated to the idea is beginning to spread. Perhaps most notable is the Books For Free project, which has used a number of empty shop spaces to offer free books in places such as Redditch, Liverpool, Wolverhampton and London's Tufnell Park.

Joining the trend is a store in a much more coveted piece of real estate, indeed London's most famous shopping avenue, Carnaby Street. Or just off it, at least. Fashion-curation students at Goldsmiths University are curating the beautiful-looking Carnaby Street Book Exchange from an unused retail space in Kingly Court. The space runs along the same rules as other book exchanges – if you take one, leave one – but they're also keen for previous owners to leave notes for new ones. Wonderful ideas, all.

Find out more: ind.pn/bookexch