How to convert a linear relation to a x/y ?

This seems a simple question but given the limited resources or the Arduino, it is not that easy...

So what I am trying to achieve is to be able to reference a linear position by calling a x/y value...
My RGB board has 40 Leds, placed in a 8X5 array... So I would like to code "turn on position X6, Y4" ...
How do I do it, since we cannot use 2 dimension arrays...

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