How to solve

Problem $01\mathrm{.}001-$ An axially loaded rod comprised of two solid cylinders

Two solid cylindrical rods $AB$ and $BC$ are welded together at $B$ and loaded as shown. Take $d_1=30mm,d_2=50mm,$ and $P=65kN.$

Problem $01\mathrm{.}001.$b $-$ Normal stress in large diameter cylinder

Find the average normal stress at the midsection of rod $BC$ : $($Input the answer with the appropriate sign.$)$

The average normal stress at the midsection of rodBC is MPa $.$