UPSC CDS Math Volume and Surface Area Questions

We have three copper spheres of radius 3 cm, 4 cm and 5 cm.
We melted these spheres into a large sphere
We know that volume of sphere is ‌

4

3

πr3
∴ Volume of three small spheres = Volume of large sphere
⇒‌

4

3

π(33+43+53)=‌

4

3

πr3
⇒27+64+125=r3
⇒r=6cm