Probability of football accumulators

[splodgecat]

I have constructed a spreadsheet stake calculator, which divides my total stake appropriately amongst a series of accumulators so that I can make a profit as long as 'x' teams win, and 'y' teams win or draw.

i.e. as long as Man Utd and Chelsea win, and Bolton win OR draw I can make a profit as follows:

Man Utd win & Chelsea win & Bolton win = stake of £65.04 = £186.15 winnings
Man Utd win & Chelsea win & Bolton win = stake of £34.96 = £186.15 winnings

(according to odds that I have made up for this example and a total stake of £100).

My question is this:

a) How do you calculate the probability of winning one accumulator bet?
b) How do you calculate the overall probability of winning EITHER the above accumulator bets?
c) How can I apply this to a larger example? i.e. winning one of 4, 8, or 16 accumulators?

Many thanks for your help. I'd like to have the spreadsheet show the probability of winning money on the bets placed.