Final answer:
To find the age of an invoice in Excel, subtract the invoice date in cell B6 from the current date in cell B3, and enter the formula =B3-B6 in cell F6.
Step-by-step explanation:
In order to display the age of the invoice in column F on an Excel spreadsheet, Brett needs to calculate the number of days between the current date and the invoice date. To do so without using any Excel functions, he can simply subtract the invoice date in cell B6 from the current date listed in cell B3. Therefore, the formula he should insert in cell F6 would be =B3-B6. This will yield the difference in days, effectively showing the age of the invoice.