15.7 (6th Ed.): Figuring out reactant rate order

[MadisonFuentes1G]

How do we know which experiments to use in order to figure out a reactants rate order? I thought it was by choosing experiments where one reactant concentration changes while the other don't, but that doesn't seem to be the case for finding [B] (the solutions uses exp. 2 and 3, where there is also a change in the conc. of reactant A).

[Janice Park 1E]

Hey! I think you're actually looking at problem 17 in the 6th edition.
The reason why we can use experiments 2 and 3 to find the order for the B is because:
The concentration of C doesn't change the rate of the reaction, as seen between experiments 1 and 4 (The concentration of C changes, but the rate doesn't change).
Therefore, if we disregard changes in concentration of C, we can see in that experiments 2 and 3, the concentration of A does not change, but the concentration of B does change, which we can then use to find reaction rate order.
Hope this helps!