How many significant digits are there in exact numbers? What kinds of numbers are exact?

Solution 6Q

Exact numbers are those which are exactly known.

Therefore, exact number can not have uncertainty in it. Thus, exact number have an infinite number of significant figures.

Exact numbers are those which have defined numbers. For example: Conversion within american system (ft in mile ,etc. )

Conversion within metric system( m to km, cm to m),