Question

the diameter of a spherical ball is 21 cm how much leather is required to prepare 5 such balls

Answer 1

d=21 cm 
r= d/2= 21/2cm
surface area of 1 sphere= 4*22/7*r*r
= 4*22/7*21/2*21/2
=22*3*21
=1386cm²
now for 5 balls = 5*1386 cm³
=6930 cm² of leather is required

Answer 2

Surface area of  sphere= 4*22/7*r²
--------------------------------------------------
D=21 cm 
--------------------------------------------------
D= d/2
= 21/2cm
---------------------------------------------------
Surface area of  sphere= 4*22/7*r²
---------------------------------------------------
= 4*22/7*21/2*21/2
-----------------------------------------------------------
=22*3*21
-----------------------------------------------------------
=1386cm²
------------------------------------------------------------
∴ 5 balls = 5*1386 cm³
   →6930 cm²
--------------------------------------------------------------
∴ leather is required to make 5 balls= 6930 cm²