Answer : The atomic radius for Ti is, 
Explanation :
Atomic weight = 47.87 g/mole
Avogadro's number 
First we have to calculate the volume of HCP crystal structure.
Formula used :
.............(1)
where,
= density = 
Z = number of atom in unit cell (for HCP = 6)
M = atomic mass = 47.87 g/mole
= Avogadro's number
V = volume of HCP crystal structure = ?
Now put all the values in above formula (1), we get


Now we have to calculate the atomic radius for Ti.
Formula used :

Given:
c/a ratio = 1.669 that means, c = 1.669 a
Now put (c = 1.669 a) and (a = 2R) in this formula, we get:



Now put all the given values in this formula, we get:


Therefore, the atomic radius for Ti is, 