Question

31base four + 13 base four =

Answer 1

$31_4+13_4$

Adding the digits in the "ones" place gives 4, which in base-4 amounts to setting equal to 0 and carrying the "one" to the "fours" place:

$1_4+3_4=4_4=10_4$

The same thing happens in the "fours" place, so you end up carrying the "one" to the "sixteens" place.

$30_4+10_4=40_4=100_4$

So putting this together, you should get

$31_4+13_4=110_4$