Apple software - Numbers
You would use a formula for this.  In the cell you want the SUM to display (BG1 in your question), enter this formula
=SUM(I1,O1,U1)

Note that the function is SUM() preceded by an equal sign = and the cell values are comma separated. You can also include ranges by putting a colon between cells.  Say you wanted to select A1 through E1, then I1, O1, and U1:
=SUM(A1:E1,I1,O1,U1)

Here is a list containing some of the functions available to you in Numbers.  Apple - Numbers - Functions