I fear you have mangled the question.
As written, you say
n+1 + 2n+1 = 2(4n)+0
sorry - n is not an integer
Now, if you meant ... = 20 base 4
then you have
n+1 + 2n+1 = 2*4+0
n=2
But in that case, 21 is not a valid base-2 number
Solve for n the equation 11base n plus 21base n equal 20base 4n
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