A defeats B by 10 metres in a 100-metre race. B defeats C by 10 metres in a 100-metre race. By what distance will A defeat C in a 100-metre race?

19 m

According to the question,

When A covers 100 m, B covers 90 m

And when B covers 100 m, C covers 90 m

So, the distance ratio of A to B and B to C are 10 : 9 and 10 : 9

A : B : C = 100 : 90   : 81

So, A beats C by 100 - 81

= 19 m

So , A beats C by 19 m