I'm building a holiday expense planner and I'm trying to get the sum of a column in a table, however as I pay things off I want to be able to strikethrough the amount and then it minus it from the total sum.
Is this possible with Numbers?
I haven't used Numbers but I would say "Probably not."
Spreadsheets don't generally expose their presentation layer (font styles, colors, etc.) readily to their calculation layer (cell formulas and inbuilt functions).
A common approach to your task is to use another column to store a status for each payment (Due, Paid, etc.) and then to use a conditional summation function to correctly sum the total. The Excel function is SUMIF and I'd expect Numbers to have something similar.
I'm not sure you could necesserily do it with a strikethrough but another approach would be to use different formats for positive and negative numbers where the neggative doesn;t show up. The various accounting formats often include on where the negative doesn;t show up but the number appears red when it is a negative.