First we need to draw this distribution and label three standard deviations on each side of the mean to determine where these weights fall. That will tell us which segments to add up. Here is our drawing of this distribution from before.
Using this Normal curve along with the diagram above we see we need to add the two segments between 78 and 92 pounds to get
47.5% percent of black labs weigh between 78 and 92 pounds.
We add all the segments below 71 pounds to get
16% of black labs weigh below 71 pounds.
We add all the segments below 64 and above 78 pounds to get
52.5% of black labs weigh below 64 or above 78 pounds. Notice we could have done this in a faster way since 78 is the mean we know the percentage above the mean is 50%. We could have added