How will you bring about the following conversions?

Bromomethane to propanone

Bromoethane on treatment with alcoholic KCN forms Acetonitrile with the removal of KBr. CN is added in the case where there is a need to increase the number of carbons. Further the acetonitrile is treated with CH₃MgBr(Grignard reagent) in the presence of ether, the intermediate so formed is hydrolysed, and the product formation takes place i.e. Propanone and NH₃ is obtained as the by-product.