The rate of disintegration of a radioactive substance falls from...

$A = {A}_{0}e^{- \lambdat}$

$⇒ 100 = 800e^{- \lambda ( 6 \times 60 )}$

$⇒ e^{- 350x} = \dfrac{1}{8}$

$⇒ - 360\lambda = \ln\left(\dfrac{1}{8}\right) = - \ln 8$

$⇒ \lambda = \dfrac{\ln\left({2}^{3}\right)}{360} = \dfrac{\ln 2}{120}$

$⇒ {T}_{1/2} = \dfrac{\ln 2}{λ} = \dfrac{\ln 2}{( \ln 2 / 120 )} = 120minutes$

$⇒ {T}_{1/2} = 2hours$