How to do it?

90/100  = x/ 10      cross-multiply

900/100 = x

9 = x

So......90 out of 100  =  9 out of 10

Your question doesn't make sense!. State it clearly.

how to convert a mark out of 90 to out of 10?

If you divide 90 by 9 you get 10

So

If you divide your mark out of 90 by 9 you will bet you new mark out of 10  :)

That is

Divide by 9

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I think CPhill misinterpreted your question. :/