£100 divided by odds(evens = divide by 2, 1/2 = divide by 1.5 etc)
At evens you'd lay £50.
Ronnie wins, you gain £80 from your bet and lose £50 from your lay, £30 profit.
Ronnie loses, you gain £50 from your lay and lose £20 from your bet,£30 profit.
Long time since I've had a bet so I would check that it works for a few examples b4 I took it as accurate.