$($b$)$ A steel pipe having an inside diameter of $800$ mm is made of two semicircular halves welded together. If the efficiency of the welded joint is $75\%$ and the allowable stress in tension for the steel is $110$ MPa $,$ what is the minimum wall thickness $($in mm $)$ required to sustain a pressure of $3\mathrm{.}8$ MPa $?$ Give the answer to $1$ decimal place.

$(7$ marks$)$