Preventing double entries in a same row, different column

[crehan57]

A B C D
1 LAST FIRST SINGLE COUPLE
2 Abernaty Pat 25.00
3 Adonis Theo 40.00
4 Arabiatta Luigi 40.00
5 Hermann Peter 25.00


Here is my problem. In the above fictitious example, I have ensured (Validation) that each cell in column C and D prevent me to enter a wrong fee amount, by forcing me to use an drop down list with the only fee accepted for the column, in this instance, $ 25.00 dollars in the C column (SINGLE) and $ 40.00 in the D column (COUPLE).
This works great.
However, it will also allow me to enter a $ 25.00 AND a $ 40.00 fee for a single individual, which is wrong since an individual can only be in the SINGLE membership or the COUPLE membership structure.
How can I prevent entering an amount in column C if column D already has an amount entered, and inversely?

Is there a simple solution to my predicament? You have no doubt deduced that I was a basic Excel user and any solution submitted must be explained in very basic steps. Thanks in advance.

[ChemistB]

How about in C have a dropdown Single or Couple
Then in D
=IF(C2="Single", 25, IF(C2="Double", 40, "")

[José Augusto]

Hi

You can use data validation with the following formulas

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for Single and

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for Couple

But I like ChemistB´s suggestion

Best regards

[crehan57]

Sorry for the delay in replying and thank you for your assistance. I have change the format of my spreadsheet and solved my problem wit the addition of another column.