How many significant figures should I solve with?

Question

Here is my question:

 

Calculate the radius of an iridium atom, given that Ir has an FCC crystal structure, a density of 22.4 g/cm³ , and an atomic weight of 192.2 g/mol and N_A = 6.022 x 10²³.

How many significant figures should I solve with?

Answer 1

The two measured values in the problem (mass and Avogadro's number) are both in four sig. fig.'s.  Density is not significant because it is a calculated value (not measured).  You already know this!