Given, dimensions of silver coins (cylindrical shape) Thickness (h1)=4mm =
4
10
cm=0.4cm Diameter =3.5cm 1∴ Radius, r=
3.5
2
=1.75cm And dimensions of cuboid are 21cm×11cm×7cm i.e., I=21cm,b=11cm and h2=7cm Let n number of coins are melted to form a cuboid. In such case, volume will remain same. ∴ Volume of n coins = Volume of cuboid ⇒n×Vone coin =Vcuboid ⇒n×πr2h1=I×b×h2⇒n=
1⋅b⋅h2
πr2h1
⇒n=
21×11×7
22
7
×(1.75×1.75)×0.4
⇒n=
21×7×7
2×(
7
4
×
7
4
)×(
4
10
)
n=
21×4×4×10
2×4
=21×2×10=420 Hence, 420 silver coins must be melted to form a cuboid.