R(Old) = [(rho) X l] / (pi) r^2

R(New) = [(rho) X 2l] / (pi) a^2 {a is the new radius}

Dividing the two We Get

R(old)/R(new) = a^2/2 r^2

In this case Volume is conserved so it can be used to relate r & a

(pi) r^2 l = (pi) a^2 2lr^2 = 2a^2

Substituting Above We get

R(old)/R(new) = 0.25

R(new) = 4 X R (old)

R(new) = 4 X 5

R(new) = 20

This is of the whole wire but as it is divided in two equal parts and resistance is directly proportional to length so the length of each piece will be 10 Ohm