JLPT N3 Reading

The shopping street by the station has a 'shared umbrella' scheme. Someone caught by sudden rain can freely borrow an umbrella from a basket outside a shop and return it later to any shop — not necessarily the one they borrowed from. When it started, some worried the umbrellas wouldn't come back, but in practice about 80% are returned. With so many places to return them, it's easy, they say. The other 20% don't come back. The scheme survives anyway because the umbrellas are supplied by donations of old ones. Q: Why can it continue even though 20% aren't returned? — The umbrellas come from donations.