you explained very well BUT, you should always start from the last digit not the first digit.... so as you said base 8 is correct ...so you get 31 seprate 3 1 ..... and then start from 1....... 1x(8^0)[power] [as 8^0 = 1, 1x1 becomes 1] and then take 3 , 3x(8^1) = 24.......
same...