The compound MO has zinc-blende closest unit cell. If the radius M²⁺ ion is 60 pm, the edge length of the unit cell of MX is

In zinc-blende closest unit cell, cations occupy tetrahedral voids for which r_c/r_a= 0.225. Hence, r_a = r_c  /0.225 = 60 pm/0.225 = 266.67 pm. Anions occupy face centered cubic positions. Hence, $4r_a=\sqrt{2} a, or a=2\sqrt{2} r_a=\left(2\sqrt{2}\right)$ =(2.667 pm) = 754.3 pm