Zero order half life Two substances A and B react with each other in such a way that A is 50% consumed in 33 minutes and 75% consumed in 66 minutes. Changing the concentration of B has no effect on the results. Which statement is true? 


And the answer is: 


  This reaction is 1st order in A and zero order in B.


I understand why A is first order but I don't get why B is zero order. The equation for the half life of a zero order reactant is $\displaystyle\frac{[\ce{B}_\text{inital}]}{2k}$.

Doesn't this mean that the concentration for a zero order reactant would affect the half life?
Changing the concentration of B has no effect on the results.

This statement could only mean one thing: B does not participate in the reaction. Obviously, if B does participate in the reaction, and you change the concentration of B to 0 the reaction won't happen.

The equation for the half life of a zero order reactant is
[Binital]2k.

Zero order reaction have no half-life. Only 1st order reaction has half-life.