The mean is the average of the numbers.
It is easy to calculate: add up all the numbers, then divide by how many numbers there are.
Let the number of sit-ups you have to do Sunday be x. Then the sum of all sit-ups is
100+65+120+150+90+0+x=525+x.
The number of days is 7.
Thus, the mean is
You know that the mean number of sit-ups per day is 100, equate these two means and solve the equation:
Answer: 175 sit-ups