jmc

A ball of yarn 12 inches in diameter has twice the diameter of a ball of yarn 6 inches in diameter. Let the radius of the small ball be $r$ and the radius of the large ball be $2r$. Then, the volume of the small ball is $\frac{4}{3}\pi r^3$ and the volume of the large ball is $\frac{4}{3}\pi (2r{)}^3=8\cdot \frac{4}{3}\pi r^3$. Hence the large ball has 8 times the volume of the small ball, so Mrs. Read can knit $\boxed{8}$ pairs of identical mittens with the large yarn ball.