Question

The circumferences of the two concentric circles are $12$ cm and $72$ cm. What is the difference between their radii? [Use $\pi =3$]

• A. $12$ cm
• B. $10$ cm
• C. $9$ cm
• D. $14$ cm

Answer 1

Answer: (B)

$10$ cm

Given circumferences of are circles are 12cm and 72cm respectively.

formula of circumference of a circle is $2\pi r$ [Here r is thr radius of a curcle]

and $\pi =3$ [given]

Circumference of first circle= $2\pi r$

$\Rightarrow 2\times 3\times r_1=12$

$\Rightarrow r_1=2cm$

Circumference of second circle= $2\pi r$

$\Rightarrow 2\times 3\times r_2=72$

$\Rightarrow r_2=12cm$

Difference between radii=$r_2-r_1=12-2=10cm$

So, option B is the answer.