Several friends traveling abroad together—this time you covered lunch in dollars, next time they'll cover shopping in Hong Kong dollars. When you get back home, it's impossible to remember who owes what in different currencies. Colleagues sharing work lunches regularly, with different people paying each time. By month's end, no one can track who's participated or who owes whom. Friends sharing an apartment—who paid utilities this month, who paid property fees next month, and by year's end ever...
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