For parallel reaction

When adding the rate constants of the two reactions, we need to use the formula for combining rate constants of two reactions in parallel, which is:
k=k1+k2
where k1 and k2 are the rate constants of the individual reactions.
In this case, the half-lives of the two reactions are 12 min and 3 min, respectively. The rate constants of the two reactions can be calculated using the formula for half-life of a first-order reaction:
t1/21=121+31=125
Net t1/2=512=2.4min≈2min
Hence time taken for 50 consumption of reactant will be close to 2min.