TO make $100 on a 2.1 return is 100 divided by 1.1 equals $91.

That loses so now have to make $191.

Next bet is target $191 and you say next return is 2.5 so that makes your odds to bet to 1.5. So $191 divided by 1.5 = $128 (rounded up ), so now you have $219 out in bets.
You get a win so that is 2.5 x $128 = $320.

$320 minus your bets of $219 = $101 profit. (your target).

You must remember when working to decimal odds you have to take off 1.

BUT after all that DON'T DO IT.