The number of Pbr3 that contain 3.68 x10^ 25 bromine atoms is 60.631 moles
calculation
by use of Avogadro law constant
that is, 1 moles = 6.02 x10^23 atoms
what about 3.65 x10^25 atoms
by cross multiplication
= (1 mole x 3.65 x 10 ^25 atoms) / ( 6.02 x10^23 atoms) = 60.631 moles
atoms and atoms cancel out each other