I strongly disagree with 50% and the coin flip analogy. With this problem the tank needs to get across multiple bridges (at least 3) to get across.Therefore, you have to take all previously crossed bridges' probabilities into account. The shortest number of bridges the tank can cross is 3, but that doesn't mean it will cross 3. I'm not sure what the math would be to determine the number of bridges crossed, but the answer is at the most 1/2 * 1/2 * 1/2 = .125, though I'm certain the answer is lower because the amount of bridges to be crossed is higher than 3 since the original question doesn't ask for the least amount. Can anyone help on the math for that? (is it really 7-3=4? and if so, why?)