Answer : 1.26
The expression of density is
$\rho =\large\frac{N}{a^3}\big(\frac{M}{N_A}\big)$
For face-centred cubic unit cell,N=4.For body-centred cubic unit cell,N=2
Hence,$\large\frac{\rho_{FCC}}{\rho_{BCC}}=\frac{N_{FCC}a^3_{BCC}}{a^3_{FCC}N_{BCC}}=\frac{4}{(3.5A^{\large\circ})^3}\frac{(3.0A^{\large\circ})^3}{2}$$=1.26$