Hi,
As minnesotaart has mentioned this approach uses a macro - a piece of Visual Basic code. Not sure how much you know about the hidden delights of Excel so apologies if I'm starting to teach Grandma to suck the proverbial eggs.
Excel has 'behind' it a rich piece of software called Visual Basic (for) Applications. Known as VBA or more commonly macros. This allows you to automate workbook processes and build whole systems of decision driven operations. At its most rudimentary you can capture VBA code by using the Macro Recorder which you'll find in the Ribbon under the Developer Tab. And if you don't see the Developer tab go to Excel options and select the Customise Ribbon to make it 'live'.
In your particular case we're using what's know as the sheet's 'double click' event code, i.e. a piece of code which runs once any cell on the sheet is double clicked. There are lots more events I just happened to choose this one as being perhaps the most obvious. You could have a right click event, a change event or others.
Here's the code.
You'll see that all the cell references are hard coded. This is generally bad practice, I did it for simplicity. It's much more efficient to give range names to specific cells like G7, G8 etc. and use the names in the code. Which is sort of obvious when you think about it since if you were to add a new row to the letter sheet above G7 this code would immediately be compromised and it wouldn't work, whereas if it use range names it would.
Anyway, what this macro essentially says if the cell being clicked (known here as the target cell) is anywhere between A2:F100 then carry on with the macro. Furthermore if its equal to Nothing then end the macro.
Assuming these two tests are satisfied then it goes on and makes cell G7 on Sheet2 (the Letter page) equal to cell Ax where x is the row number of the cell you happen to have double clicked.
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