To predict the theoretical yield we would have to know the amount we of product we actually produced in a reaction, and the amount that should have been produced based on calculations involving the balanced equation for the reaction.
For instance, suppose we made water from hydrogen and oxygen. Starting with 2 moles of hydrogen and 1 mole of oxygen we should produce 2 moles of water (or 36 gram). But suppose we actually ran the reaction in the lab and collected only 34g of water.
The percent yield would be 34g / 36g x 100 =94.4%
So, if I'm reading your question correctly, 20g is the experimental yield and 10% is the percent yield.
Plugging in gives us 10/100 = 20/x
The theoretical yield must be 200g.
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I HOPE THAT HELP MY FRIEND :)