Why do you change the sign when subtracting a negative number?

Asphere has a diameter of 10 in. what is the volume of the sphere?

The median of a data set is the measure of center that is found by adding the data values and dividing the total by the number of data values that is the value that occurs with the greatest frequency that is the value midway between the maximum and minimum values in the original data set that is the middle value when the original data values are arranged in order of increasing (or decreasing) magnitude

65% of students in your school participate in at least one after school activity. if there are 980 students in you school, how many do not participate in an after school activity?