Mr. BB, the question asks for probability, not odds.
Your equation
nCr =20C3 =20! / {[20 - 3]!. 3!} =1,140 to 1
is SLOP!
Why would you expect any student to understand this unconventional hodgepodge of SLOP?
In this case, nCr stands for “no Crap required.” None is required, because you’ve maxed it out!
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Sir CPhill presents a coherent solution here.