It is not good enough to know (and convey) the precision of measurements. We will almost always have to do something with those numbers, we therefore need to consider how is our precision affected by arithmatic manipulation.
Determine the number of significant digits both in a given value as well as in the result of arithmetic
How many significant digits are there in 2.04x10-3?
Your Car weighs 2100 lbf
You put an 8 lbf bag in the trunk
What is the car's weight now? [cheat]
You measure a samples mass to be 3kg
Calculate the samples weight (multiply by gravity: 9.81 m/s2)
What is your answer? [cheat]