A step tracer input was used on a real reactor with the following results: For t 10 min, then CT = 0 For 10 t 30 min, then Cr = 10 g/dm For t 30 min, then C = 40 g/dm The second-order reaction A B with k = 0.1 dm/mol-min is to be carried out in the real reactor with an entering concentration of A of 1.25 mol/dm at a volumetric flow rate of 10 dm/min. Here k is given at 325 K. (a) What is the mean residence time tm? (b) What is the variance