An optically active substance decomposes into optically active substances ‘B’ and ‘C’  as follows
 $A\stackrel{k=0.001\min ut{e}^{-1}}{\to }2B+C$
 The  specific rotations of A,B and C are $+{40}^0,+{10}^{0}and-{30}^0$   per mole, respectively. If initially A and C were present in 4:3 mole ratio, the time (in min), after  which the sample becomes optically inactive is  $\left(\ln 2=0.7,\ln 5=1.6,\ln 7=2.0,\ln 13=2.5\right)$