25 in base 10 (Dec) is the same number as 31 in base 8 (Oct)
In base 10, the "units" is the number of 10^0, the "tens" is the number of 10^1, the hundreds is the number of 10^2 and so on. So, for example, 745 is (7x10^2) + (4x10^1) + (5x10^0). In base 8 it's the same, but replace the 10s with 8s. So 31 in base 8 is (3x8^1) + (1x8^0)= 24+1 = 25 (in base 10, which is what we work in)
EDIT: Similar kind of thing to: "There's only 10 types of people in the world, those who understand binary [base 2], and those who don't."
Heisenberg and Schrodinger are driving along a road in a car. A policeman pulls them both over and he asks Heisenberg "Do you know how fast you were travelling back there?
Heisenberg replies "No, but I know exactly where I am"
The policeman takes this answer as a cue to search their car, and starts in the boot.
After searching for a few minutes, he asks Schrodinger "Did you know you have a dead cat in your boot?"