When we are doing addition and subtraction with measured quantities, the process is quite simple, but it looks a bit complicated when we write it down. First, we look at the rule and then there are some examples to show you that it is not as hard as you might think!
Rule 4: For addition and subtraction, write down the measurements as though you were going to do the calculation by hand, with all of the decimal points line up under each other (and with none of the numbers in scientific notation). Look for the number that has the least number of decimal places in it. That is how many decimal places will be in the answer. 
That looks pretty painful, doesn't it? Well, it is not all that hard. Let's see some examples:
Example 1  Example 2  



In the first example, the quantity with the least number of decimal places is the second one (2.13). Consequently, our answer will should be rounded off so that it only has two figures after the decimal place.
The second example is similar; the second quantity (6.50) has only two significant decimal places, so the answer will have a similar amount of accuracy and rounding off is used to reduce it to the correct number of figures.
To summarise,
Example 1  Example 2  

Answers:  17.88  30.94 
If you are still a bit confused about this, you can try the exercises at the end, or look at another example.