2100-2105 puts spread value = 1.80 1.80/5 = 0.36... I think @sle means that there's a 36% chance of the putspread becoming ITM? Correct @sle ? Since your risk is 1.80 and max profit is 5... so ... probability theory and risk/reward...
Exactly. This is actually a better measure of probability at expiry then delta and is just as easy to calculate. That's how one would price a European digital bet and it gives a true representation of the markets view on probability (obviously, including the skew and kurtosis).