To subtract 7 from 13 you use the fact that 13 is one ten plus three units, i.e. 13 = 10 + 3. then you can subtract 7 from 10 which is 3, and then add the 3 other 3 units to get 3 + 3 = 6. In this way, you have subtracted 7 from 13 using the fact that 13 is one ten plus 3 units.
