How do I convert 90% into a fraction showing your work

Your answer is 9/10

if you have a pie that is cut into 10 pieces, but nobody has taken a piece yet, then it is 100%. You return an hour later and one piece has been taken you are now at 90% of the pie. The pie is now 90% out of 100% or 9 slices out of 10. 9/10