The Egyptians operated a base 10 number system without place value. Addition was achieved by collecting like symbols together and converting every 10 of them into 1 of the next size symbol:

Subtraction is essentially the removal of the required number of each symbol. This is, however, complicated when more of a symbol are to be taken away than there are present.

Consider this calculation: 63 - 38

From the 6 tens can be taken 3 tens but only 3 units can be removed. This leaves 5 units still to be subtracted.

One of the remaining tens is needed to enable the further 5 units to be subtracted i.e. 1 ten - 5 units=10 units - 5 units=5 units

The exact mechanism that was used is not clear though this illustration shows how the scribe may have envisaged the calculation: