for example, 7426 in octal minus 4755 in octal, write the equation directly, 7426 - 4755, then 6 minus 5 = 1, 2 is smaller than 5, borrow 8, 8-5+2=5, the same reason, 4 is borrowed before it has passed 8, so subtracting 1 equals 3, borrowing 8 again, 8-7+3=4, 7 borrowing 1 becomes 6, 6-4=2, the result is 2451