If you are of a certain vintage you may have been taught to carry out subtraction sums using the "borrow and pay back" method. Whilst this method led to the correct answer it was not logical as to why it worked.
More recently subtraction is taught using the "renaming" method.
So, say you have the sum 43 -17.
Starting with the units column (right most column) you find that 3 subtract 7 cannot be done.
So you go next door to the tens column. You take one of the tens and convert it to units. You now have three tens left so cross out the 4 and write in a 3 beside it.
You bring the ten over to the units column. You now have the three original units plus the ten units you brought over from the tens column, resulting in 13 units in total. Add a little 1 beside the 3 to signify the 13 units.
Now you return to your units column where you find that 13 subtract 7 is 6.
Next move on to the tens column where you find that 3 tens subtract 1 ten is 2.
Your final answer is 43 - 17 = 26.
Still not sure? Just drop in and we will go through it for you.