Problem $3.$

$8\mathrm{.}36$ A cold$-$drawn rectangular steel bar of $140$ Bhn is $10$ mm thick, $60$ mm wide, and has a central hole $12$ mm in diameter $($as in Figure $4\mathrm{.}40).$ Estimate the maximum tensile force that can be applied to the ends and have infinite life with $90\%$ reliability and a safety factor of $1\mathrm{.}3$ : $($a$)$ if the force is completely reversed and $($b$)$ if the force varies between zero and a maximum value.

Please help..assume Sn$=135$Mpa Kf$=2\mathrm{.}28$