smc

When the circumference of a circle is increased by a percentage, the radius is also increased by the same percentage (or else the ratio of the circumference to the diameter wouldn't be $\pi$ anymore) We see that the circumference was increased by $25\%$. This means the radius was also increased by $25\%$. The radius of the original balloon is $\frac{20}{2\pi }=\frac{10}{\pi }$. With the $25\%$ increase, it becomes $\frac{12.5}{\pi }$. The increase is $\frac{12.5-10}{\pi }=\frac{2.5}{\pi }=\boxed{\frac{5}{2\pi }\text{ in}}$.