All digits in a number are significant except
In counted numbers like twelve eggs or numbers defined as equalities such as `1\ "km" = 1000\ "m"`, the number of significant digits is infinite.
When you add or subtract numbers, the answer should have no more decimal places than the given value with the fewest. When you do anything else, the answer should have no more significant digits than the given value with the fewest.
We use scientific notation for small numbers and big numbers. It is also useful for making it very clear how many significant digits a number has. For example, we would assume that 100 has one significant digit. To show that there are three, you have to use 1.00 × 102.
Make sure you remember all the metric prefixes. Also, remember that you can multiply by 3.6 to convert a speed from m/s to km/h (and divide to go the other way).