If you have a large amount of data that you need to find the sum of, rather than doing =SUM(C3+C4+C5) etc. there is a much quicker way to manage your data.
By using the formula =SUM(C3:C21) you are automatically including every cell between and including C3 to C21.
Then press enter and your sum will be displayed in the selected cell.
By changing one of the values within the selected range, the sum is automatically updated without the need for you to update anything.