20₃ - 11₃ = 20₃ - 10₃ - 1₃
… = 10₃ - 1₃
… = 2₃
Put another way, we "borrow a 1" from the 3¹ digits place:
20₃ = 1(3)₃ - 11₃ = 2₃
But this is just the number 2 (in base 10), which in base 2 would be 10₂.
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