Here is a quick tip- for double underlining numbers.
Currently to double underline a number you might:
Go to the Format menu, then select Cells, then choose Double Underline from the dropdown list
or
Choose double underline from the ranges of choices from the Borders box in the formatting toolbar
The first choice just underlines the number.
The second choice will put a line above and double lines below going across the entire cell, no matter how long you have the number.
Here is a faster way to do the first option.
Hold the shift key
Click the Underline button on the Formatting toolbar
This saves you from having to go to the Format menu, etc. as mentioned above. You can do just the same with the shift key and one of the format buttons.
In case you wondered, the underline button in the Formatting toolbar is the only button we have seen with the double life (one line if regular click, double line if clicked while holding down the shift key).