Arturo O. answered 03/26/18
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Use Kepler's 3rd law.
T = period of orbit
a = average distance from Sun
T2/a3 = constant for all planets in our solar system
For the Earth,
T2/a3 = (1y)2/(1 AU)3
For Neptune,
T2/a3 = (165 y)2 / a3
T2/a3 must be the same for the Earth and Neptune, so
(165 y)2 / a3 = (1y)2 / (1 AU)3
a = (165)2/3 AU ≅ 30.1 AU