What is the half life of a first order reaction if rate constant is $$4.2 \times 10^{-2}$$ per day?

The reaction, $$3 \mathrm{ClO}^{-} \rightarrow \mathrm{ClO}_3^{-}+2 \mathrm{Cl}^{-}$$ occurs in two steps:

i. $$\quad 2 \mathrm{ClO}^{-} \rightarrow \mathrm{ClO}_2^{-}+\mathrm{Cl}^{-}$$

ii. $$\quad \mathrm{ClO}_2^{-}+\mathrm{ClO}^{-} \rightarrow \mathrm{ClO}_3^{-}+\mathrm{Cl}^{-}$$,

the reaction intermediate is:

Calculate the rate constant of first order reaction if the concentration of the reactant decreases by $$90 \%$$ in 30 minutes.

The rate law for the reaction $$\mathrm{A}+\mathrm{B} \rightarrow$$ product is given by rate $$=k[A][B]$$ Calculate $$[A]$$ if rate of reaction and rate constant are $$0.25 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{~s}^{-1}$$ and $$6.25 \mathrm{~mol}^{-1} \mathrm{dm}^3 \mathrm{~s}^{-1}$$ respectively $$\left[[\mathrm{B}]=0.25 \mathrm{~mol} \mathrm{dm}^{-3}\right]$$