Finkelstein reaction is an example of 

Acetone is a polar aprotic solvent. It favors ${\mathrm{SN}}^2$ reaction.

order of reactivity 

${\mathrm{CH}}_3\mathrm{X}>1^{\mathrm{o}}\mathrm{RX}>2^{\mathrm{o}}\mathrm{RX}>3^{\mathrm{o}}\mathrm{RX}$