MENSURATION

Mass = density X Volume

Density of a material is  same let it be k
let V1 be the volume of smaller sphere and v2 be the volume of larger sphere
So  ATQ
kV1 = 1 kg (i)
KV2 = 7 kg  (ii)
dividing (i) by (ii) we get  v1/v2 =1/7  Now radius of  smaller sphere is 3 cm and let it be r for bigger sphere
3³ / r³  = 1/7
r³  = 81  X  7
spheres are melted to form  a bigger sphere,Let R be the radius
4/3 R³  = 4/3  ( 81 + 81  X  7 )  Solving we get R = 6 so  diameter =12 cm