Which of the following include exact numbers? a) the speed of light in a vacuum is a physical constant; to the nearest m/s, it is2.99792 ×108 m/s. b) the density of mercury at 25°c is 13.53 g/ml. c) there are 3600 s in 1h. d) in 2003, the united states had 50 states.

Explanation:

Exact numbers are those numbers which are known with 100% certainty  or accuracy . Numbers having decimal are generally not exact because they are a part of long drawn decimal number after rounding off  to certain places after decimal.

On the basis of this definition we can say as follows

speed of light is 2.99792 ×108 . Here the number quoted is not an exact number because it is a rounded off figure of speed of light to five digits  after decimal .

Similarly density of mercury too is rounded off to two digits after decimal

so it is also not a exact number.

There are 3600 s in 1 h . Here 3600 s is  an  exact number because one hour has been defined as 3600 s . It is not measured by any instrument.

In 2003, the United States had 50 states. Here 50 is an exact number .  

when it gains or loses electrons