Help with maths formula for calculating stakes

[robmatey]

Hello. I need some help working out a formula for the following. 

I need to know how to work out the stake I need to place to make a fixed profit taking into consideration the current and previous stakes, as well as the odds for the bet. i.e. say I want to make 100 units profit but I have lost the first three bets in a sequence of bets, how much do I need to bet to make back all of my stakes plus 100 units profit. For example:

The odds are 3.2, I've staked 175 units so far, how much do I need to bet to recoup all of my previous stakes and the current stake, and make 100 profit?

I know there is a formula for this but I can't work it out and my Excel skills are not what they used to be

Any help much appreciated. 

[Valiant Thor]

(175 + 100) / (3.2 - 1) = 125 stake