The formula for mass of the sphere m =Volume x density
=4/3 (pi) (r)^3 x density
m1= 160g, r1=8cm.
r2=4cm, m2=?
160=4/3 (pi) (8)^3x density-----(1)
m2=4/3 (pi) (4)^3x density-----(2)
now equation (2)/(1)
m2/160= (4)^3/(8)^3
m2=(4/8)^3 x 160
=160/8
=20 gm
mass=20g.... mass is related to volume (if the density won't object :p ) so its smthn lyk ---> V1/V2 = M1/M2 volume is in r^3 & others are similar in the fraction, so --- (r1)^3 /(r2)^3 = M1/M2 whch mns 4*4*4/8*8*8 = M/160 finlly M=20g