I am solving an exponential distribution question and I am confused with the given rate parameter. The exponential distribution question is in below:
Suppose that the length of a telephone call (in seconds) is exponentially distributed with rate parameter $r = \frac{1}{187} {\rm s}^{-1}$ . Find the interquartile range of a call length
As I understand the exponential distribution rate parameter is $\frac{1}{\lambda}$. However, the given question is $\frac{1}{\lambda}^{-1}$ . May I know what does it mean? I try to search more information about this but I cannot find it from any elsewhere.