The circumference of the circle is given by C = 2 π r
If the radius of the circle is doubled, then new radius will be r' = 2r.
And new circumference will be C' = 2·π·(2r) = 2·(2 π r) = 2·C
It is clearly visible that new circumference is twice the original circumference i.e. C' = 4 π r
