In a 100-metre race, A beats B by 10 m and A beats C by 19 m. By how many metres does B beat C in a 100-metre race?

Explanation

When A runs 100 m, B runs 90 m and C runs 81 m, so C covers 81 / 90 of B's distance; when B runs 100 m, C runs 90 m and B beats C by 10 m.