In the year 830, *Mohamed Ben Musa Alkarismí*, one of the most notable sages of the 9th century, subtracted whole numbers as follows:

*(So you can keep track of operations, we'll use modern digits here.)*

From 12025 let's take 3604.

The operation was started from the left (**operation I**). So the 12 took 3 and 9 left; canceled the digits considered (12 and 3) and wrote the rest obtained on top of the "minuendo".

Continuing: at 90 it took 6 left 84. The difference obtained (**operation II**) was written about the "minuendo" and the digits that formed the subtraction terms were canceled.

Finally, 8425 took 4 and 8421 remained (**operation III**).

And so we have the difference between the given numbers.