Assuming the numbers 1 + nn =100 were added in base n + 1

n + 1 = n +1 and n + 1 in base n + 1 = 10, write 0 and carry 1, which will be added to the second n, resulting in another n + 1 in base n + 1 = 10.

So, putting the result together 1 + nn = 100 provided, the addition was carried out in base n + 1.