Using numbers and handling data

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# 1.9 Addition of decimal numbers

If we add 109.8 ml of one liquid to 6.5 ml of another liquid, what would be the total volume of liquid in ml?

To compare 109.8 with 6.5, you need to remember that

Place the two numbers in a grid on top of each other and make sure that columns representing the same magnitude line up with each other, and add an extra line at the bottom where you can put the result of the addition. You should end up with something that looks like this:

To add these numbers start from the right, with the smallest sized group and compare the numbers in the Tenths column.

In the decimal system, ten of one group must be expressed as one of the next sized order of magnitude, so 13 tenths is expressed as one unit and three tenths.

Then move up one order of magnitude to the next column to the left (Units) and add these values together.

Add the five units here to the one that's already in the result line, giving six units, and put the single ten into the Tens box in the results line.

Moving up one order of magnitude and adding the tens together we have

There are no tens to add to the result line, but you keep the 1 that you've already put there from the addition of the units.

Move up to the final order of magnitude and add the hundreds together.

So, as you can see from your results line, the sum of 109.8 ml and 6.5 ml = 116.3 ml

Now practise some additions for yourself with practice question 12.

Right click and open the practice questions [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] in a separate window, then you can switch easily between the course text and the questions.

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