Q.How is methyl cyanide obtained from acetamide ?

Amides are converted to nitriles by dehydration; a strong dehydrating agent like $P_2O_5$ removes a molecule of water from acetamide to give methyl cyanide.

Reaction: Acetamide ($CH_3CONH_2$) is heated with a dehydrating agent — commonly phosphorus pentoxide ($P_2O_5$), though $ZnCl_2$ or $POCl_3$ can also be used — which abstracts a molecule of water from the amide:

$CH_3-C(=O)-NH_2 \xrightarrow[\text{heat}]{P_2O_5} CH_3-C\equiv N + H_2O$

The product is methyl cyanide, also called acetonitrile ($CH_3CN$), a nitrile.

Mechanism (brief): $P_2O_5$ is a powerful dehydrating agent; it removes the elements of water ($-H$ from N and $-OH$ equivalent from the carbonyl oxygen) from the amide functional group $-CONH_2$, converting the $C=O$/$C-NH_2$ system into the $C\equiv N$ (nitrile) triple bond, with loss of $H_2O$.

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