Assume that the weight loss for the first month of a diet program varies between 6 pounds and 12 pounds, and is spread evenly over the range of possibilities, so that there is a uniform distribution. Find the probability of the given range of pounds lost. Between 8 pounds and 11 pounds
Well to make it as simple as possible we'll assume that distribution is one pound. So out values are 6, 7, 8, 9, 10, 11 and 12. A total of 7 possible amounts. Out of those 7, 4 of them are between 8 and 11(8, 9, 10 and 11). Therefore the probability of losing between 8 and 11 points is 4/7. This would change if your distribution was say 2 pounds, or half a pound.
Adonai is absolutely correct if you have a discrete distribution with weight losses of either 6 or 7 or 8 or ... 12 pounds.
He also correctly states that you need to alter your answer if the losses have a step of 2 pounds each, such as either 6 or 8 or 10 or 12, or 1/2 pound each, such as either 6 or 6 1/2 or 7 1/2 or ... or 12 pounds. And you solve these cases in exactly the way that he solved it for a step of 1 pound.
However, if you have a continuouts distribution, one that allows every possible loss (using every possible fraction) between 6 and 12 pounds, then find the distance between 6 and 12 pounds (12 - 6 = 6) and find the distance between 8 and 11 (11 - 8 = 3), then divide this sub-distance by the total distance: 3 / 6 = 1/2 = 0.50, giving a probability of 0.50.