Solution:
The major axis is the sum of the perihelion and aphelion distances so it is equal to
(0.1 + 0.4) = 0.5 AU
and the semi-major axis is half of the major axis,
= half of (0.1 + 0.4) = half of 0.5 = 0.25 AU.
From Kepler’s 3rd law, P2 = a3 = (0.25)3 = 0.015625 years.