For a passing grade - the easy way to calculate it is by calculating the chance of failing and then subtracting it from 1. To fail, you need to get the first two questions wrong (3/4 times 3/4) and then get them both wrong again on the second try (2/3 times 2/3 since you are choosing from the remaining 3 choices for each). So 3/4 x 3/4 x 2/3 x 2/3 = 1/4 or 25%, so you have a 75% chance of passing.