Input in cell 1, output in cell 2; input in cell 2, output in cell 1

Hello,

See attached spreadsheet.

I am trying to create a scenario where I am able to input into cell B3, and output into cell D3 (B3*D2). HOWEVER, I'd also like the ability to input into cell D3 and output into cell B3 (D4/D2). Is this possible? If so, can I do it without macros/VBA. If macros/VBA is needed, can you help? I do not know macros/VBA.

TIA!