When the concentration of alkyl halide is tripled and concentration of OH- is reduced to half, the rate of SN2 reaction increased by
As learnt in
SN2 Substitution Nucleophilic bimolecular -
These reactions proceed in only one step.
By increasing concentration of R-X 3 times and (OH-) reduced to half, the rate of SN2 reaction increased by 1.5 times