The area of a ring whose outer and inner radii are respectively $20 \, cm$ and $15 \, cm$ is? (in $cm^2$)

If every side of a triangle is doubled,the area of the new triangle is $K$ times the area of the old one. $K$ is equal to

$A$ square is inscribed in a circle whose radius is $4\, cm$. The area of the portion between the circle and the square is

$A$ rope,by which a horse is tied,is increased from $12 \ m$ to $23 \ m$. How much additional ground will it be able to graze? (in $m^2$)

$A$ wire when bent in the form of a circle encloses an area of $1386 \, cm^2$. What will be the enclosed area when the same wire is bent into the form of an equilateral triangle?

The radius of a wheel is $1.4$ decimetre. How many times does it revolve during a journey of $0.66$ km?