Fresh catalyst is loaded into a reactor before the start of the following catalytic reaction. The catalyst gets deactivated over time. The instantaneous activity a(t), at time t, is defined as the ratio of the rate of reaction −r′_A(t) (mol·(g cat)⁻¹·hr⁻¹) to the rate of reaction with fresh catalyst. Controlled experimental measurements led to an empirical correlation:
−r′_A(t) = −0.5 t + 10
where t is in hours. The activity of the catalyst at t = 10 hr is _______ (rounded off to one decimal place).