I think they used the wrong formula and assignment for initial and final lengths. Please ask an expert to answer this and provide an explanation. thanks

A cylindrical rod has diameter of $2\mathrm{.}5$mm and length of $200$mm$.$ In the tensile test the load of $2000$N is applied. If the diameter decreases to $2\mathrm{.}2$mm$,$ determine:

a$)$ The engineering stress at this load

b$)$ The true stress and strain at this load

c$)$ The final length of the rod.