is called the perimeter of a circle.

Find the diameter $d$ (in $cm$) of a circle when its radius $r$ is

$4 \mathrm{cm}$

Find the diameter $d$ (in $cm$) of a circle when its radius $r$ is

$5 \mathrm{cm}$

Find the diameter $d$ (in $cm$) of a circle when its radius $r$ is

$7.5 \mathrm{cm}$

Find the diameter $d$ (in $cm$) of a circle when its radius $r$ is

$14.5 \mathrm{cm}$

Find the radius of a circle when its diameter is

$6 \mathrm{cm}$

Find the radius of a circle when its diameter is

$8 \mathrm{cm}$

Find the radius of a circle when its diameter is

$18 \mathrm{cm}$