A steel rod $25$ mm diameter passes centrally through a copper tube $30$ mm

internal and $40$ mm external diameter. The ends of the steel rod are threaded, and it is

fitted with nuts and rigid washers. The nuts are tightened until the steel rod is under a

tensile stress of $20$ MPa, the temperature is then raised by $50$o

C$.$ Find the final stresses

in the steel and in the copper