Assuming the first number is in cell A1 then right-click in column B to the right of it. That's, cell B1.
Select 'Format Cells' from the context menu.
Choose 'Number' tab then choose 'Number' in the list.
Change the 'Decimal places' to zero '0'. This formats the cells contents as a 'number' to '0' decimal places (a whole number).
In that same cell type (without quote marks) "=A1*0.9" and press 'Enter' key.
This gives you your first whole 90% number in cell B1. Cell B1 is now formatted and contains the formula you need to be 'copied' to other cells below it. I say 'copied' but in actual fact the cell references will get changed during the 'dragging' procedure that follows.
Now select cell B1 and move your cursor to the bottom right of that cell and you will see a cross (+) appear. When you see this cross (+) hold the left click on it and drag the cursor (keeping the left-click pressed) down through all the rows you want the formatting and formula to be 'copied' to. You should see all the 90% 'whole' numbers appear in column B.
Hope it helps.