Hello Ian

Welcome to Math Help Forum!Let me unpack this for you:means there are:there areni(>0)horses atSi (i=1,2,3...,k)withS1<S2....Sk.horses at odds ofand the numbers are in ascending order of size: .

horses at odds of

...

horses at odds of

Then the formula means that:

Does that make it clearer?

Grandad