"Previous columns" refers to the columns to the left of the column in which you are making an entry.
Line 22 column (b) = Line 27 column (a)
Line 22 column (c) = Line 27 column (a) + Line 27 column (b)
Line 22 column (d) = Line 27 column (a) + Line 27 column (b) + Line 27 column (c)
TurboTax does this for you.
