Heat and Mass Transfer

In the Guggenheim-Anderson-de Boer (GAB) sorption isotherm equation $m = \frac{m_0 C K a_w}{(1 - K a_w)(1 - K a_w + C K a_w)}$, if the monolayer moisture is $m_0 = 0.05\text{ g/g}$, $C = 20.0$, $K = 0.80$, and at water activity $a_w = 0.50$, calculate the term $K a_w$.