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When we are asked to do a calculation for either addition, subtraction, multiplication or division, we will often decide whether to carry out the calculation ‘mentally’ or to use a ‘formal’ written method.

It is important to practise both types of calculations and understand when a mental method can provide a powerful shortcut – by just being able to write down the answer - or whether the numbers involved require us to choose an effective written method involving ‘pencil and paper’, where we set out our working in columns based on the value of each digit.

Let's look at some examples.

What is 2.63 + 4.15?

Set out the calculation, making sure that each digit is lined up in the correct column based on its place value and that the decimal points are directly underneath one another.

Starting with the column with the least place value on the right, i.e. hundredths in this example, we say:
3 hundredths plus 5 hundredths is 8 hundredths,
6 tenths plus 1 tenth is 7 tenths and
2 ones and 4 ones is 6 ones.

What is 6.58+7.29?

What is 8.65+9.78?

Set out the calculation, making sure each digit is in the correct column based on its place value and that the decimal points are directly underneath one another.

Starting with the hundredths column, 5 hundredths plus 8 hundredths is 13 hundredths, which is the same as 1 tenth and 3 hundredths.

Moving to the tenths column, 6 tenths plus 7 tenths plus the ‘extra’ tenth is 14 tenths, which is the same as 1 one and 4 tenths.

Moving to the ones column, 8 ones plus 9 ones plus the ‘extra’ one is 18 ones, which is the same as 1 ten and 8 ones.

What is 6.268 + 7.894?

Set out the calculation, making sure each digit is in the correct column based on its place value and that the decimal points are directly underneath one another.

Starting with the thousandths column, 8 thousandths plus 4 thousandths is 12 thousandths, which is equivalent to 1 hundredth and 2 thousandths.

Moving to the hundredths column, 6 hundredths plus 9 hundredths plus the ‘extra’ 1 hundredth is 16 hundredths, which is equivalent to 1 tenth and 6 hundredths.

Moving to the tenths column, 2 tenths plus 8 tenths plus the ‘extra’ tenth is 11 tenths, which is equivalent to 1 one and 1 tenth.

Moving to the ones column, 6 ones plus 7 ones plus the ‘extra’ one is 14 ones, which is equivalent to 1 ten and 4 ones.