We have to define an interval about the mean that contains 75% of the values. This means half of the values will lie above the mean and half of the values lie below the mean.
So, 37.5% of the values will lie above the mean and 37.5% of the values lie below the mean.
In a Z-table, mean is located at the center of the data. So the position of the mean is at 50% of the data. So the position of point 37.5% above the mean will be located at 50 + 37.5 = 87.5% of the overall data
Similarly position of the point 37.5% below the mean will be located at 50 - 37.5% = 12.5% of the overall data
From the z table, we can find the z value for both these points. 12.5% converted to z score is -1.15 and 87.5% converted to z score is 1.15.
Using these z scores, we can find the values which contain 75% of the values about the mean.
z score of -1.15 means 1.15 standard deviations below the mean. So this value comes out to be:
150 - 1.15(25) = 121.25
z score of 1.15 means 1.15 standard deviations above the mean. So this value comes out to be:
150 + 1.15(25) = 178.75
So, the interval from 121.25 to 178.75 contains the 75% of the data values.
