One of the problems with the written method of subtraction is that there are several little variations in the process, which, if children do not understand what they are doing, will mean that they will get the answer incorrect. That is why so much time is spent with children in earlier years building up to the standard written method. Of course, if children do not know number bonds (e.g. 9 - 4 = 5) off by heart, this whole calculation will take much longer and they are quite likely to try to take short cuts and end up with wrong answers. There are several processes: 1. Looking at subtracting 3-digit numbers, the simplest subtraction is where the digits in the hundreds, tens and units on the bottom line of the question are all smaller than the corresponding digits on the top line. e.g. 765 234 This is quite straightforward: 5 - 4; 6 - 3 and 7 - 2. 2. It becomes harder when one of the digits on the bottom line is larger: e.g. 872 654 The problem arises with 2 - 4. Here an adjustment has to be made from the tens to the units. Some children will ignore this, reverse the numbers (4 - 2) and arrive at 2 as the answer. 3. In a similar way there will need to be an adjustment from the hundreds to the tens in a question such as this: 427 244 4. The hardest type is where an adjustment has to be made from the tens to the units, but there are no tens. In this case the adjustment needs to go from the hundreds to the tens and then on to the units. e.g. 305 177 Confused? Why not look at all our subtraction worksheets as well as the subtraction games for years 4 5 and 6. Go to More  3-digit Subraction