Question 965295
What is the question?
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The only question you asked it "Does this seem adequate?". Sound like yes or no should be the response, not 77.05%.
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If you have 11% of the general population being left handed, you'd assume 11% of the class was left handed. 
You can only choose an integer number of left handed desks.
3 desks would mean 3/30 or 10% are left handed. 
4 desks would mean 4/30 or 13.3% are left handed.
Choosing four seems adequate since it gives you a little extra room above the 11% whereas 3 desks leaves you shy of the average 11%.
So yes, adequate.