The root mean square speed of CH4 at 160 K is 5.0x102 m/s. At what temperature will the root mean square speed of C2H6 be the same?

u(rms) = sqrt(3RT/M)for CH4 = 5.0E2

5.0E2 = sqrt(3RT/M)for C2H6
Substitute R and M, and solve for T.