Carrying digits

When using column multiplication, mistakes may indicate a lack of understanding of the purpose of carrying digits, or an insecure understanding of place value, particularly 'teen' numbers. The similarity between carrying in addition and multiplication can be reinforced and further work on place value carried out as necessary.

For example, in this calculation, the units have been correctly multiplied to give a total of 28, but the 2 tens has been written in the units column and the 8 units in the tens column. The pupil needs to read the number from left to right, to realise that it reads 82, rather than 28.

A pupil may carry a digit correctly, but then may add it incorrectly. Here, the pupil knows that the 2 actually means 2 tens, or 20, so after working out 4 x 1 = 4, they then add 20 to this, instead of just 2. They need reminding that they are actually working out 4 x 1 ten = 4 tens, so adding 2 tens gives an answer of 6 tens.

A different mistake is for the pupil to carry correctly, but then because they multiply 1 by 4 to get the next answer, they also multiply the carried digit by 4 before adding it to the answer. So, instead of just adding on the carried 2, they add on 4 x 2. They need to understand that the carried digit is just extra tens to be added onto the answer.

Click here to generate a worksheet to practise column multiplication.