Problem $4$: The boundary layer thickness at a distance of $1$ m from the leading edge of a flat plate kept over zero angle of incidence to the flow direction is $1$ mm $.$ The velocity outside the boundary layer is $25\frac{m}{sec}.$ The boundary layer thickness at a distance of $4$ m is $($i$)4$ mm $,($ii$)2$ mm $,($iii$)1$ mm $.$