# Odds and expected value in betting

I'm reading an article about expected value in betting, I got stuck at the very beginning:

Let’s use a coin toss as an example of calculating expected value. Assuming the coin and the toss are fair, each outcome (heads or tails) has an equal probability of 50% - therefore the odds offered on a fair market would be 2.0.

This would result in an EV of 0 for either a Head or Tail - because the probability of the two outcomes is the same, so if you tossed a coin infinitely it would theoretically end up all square.

I think odds = p / (1 - p) so in this case, where the probability of landing head or tail is 0.5 so the odds should be 0.5 / (1 - 0.5) = 1? And the next point,

If however you were offered odds of 2.15 for the coin to land on heads, this is a value bet.

If you placed £10 on the coin landing on heads at 2.15, the EV is calculated likewise:

(11.50 X 0.5) – (10 X 0.5) = 0.75 This shows an EV of 0.75. Therefore you would expect to make an average profit of 75p for each £10 bet, because the odds received are better than the implied odds of the coin toss.

How is the expected value actually calculated? Where is that number "11.5" from?