‘X’ on reaction with Grignard’s reagent followed by hydrolysis gives ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}{\text{CHCH}}_{\text{2}}\text{CHO}$. X is 

If Z = – H, we get a aldehyde

If Z = – R, we get a ketone

Irrespective of the length and size of aldehyde to be prepared from RMgX, the other starting material is always HCN

${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}{\text{CHCH}}_{\text{2}}\text{MgBr}\underset{{\text{H}}_{\text{2}}\text{O}}{\overset{\text{HCN}}{\to }}{\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}{\text{CHCH}}_{\text{2}}\text{CHO}$