How do you express the sum of 9.78 g and 7.0 g using the correct number of significant digits?
The answer is written using two significant digits so the answer is
The answer can only have the same number of significant digits as the weakness link, the measurement that is the least accurate.
In this problem 9.78 has three significant digits and has been measured to the nearest .01, or 1/100 (one hundredth place) while 7.0 has only two significant digits and has been measure to only the nearest 0.1 or 1/10 ( the tenth's place)
This means that the weakness link is 7.0. The answer will only be accurate to the nearest tenth's place (0.1)
Add the the two values and round off to the nearest 0.1 or tenth's place.
16.78. this rounds off to 16.8
Remember in this problem you are adding not multiplying. So the point of accuracy is not the number of significant digits but the value
of the significant digits that have been measured.