(a) The mass of the Sun is 

   2.0 * 10^30 kg
   -------------- = 1050 times greater than the mass of Jupiter.
   1.9 * 10^27 kg

Reasoning roughly, the Sun-Jupiter system will have its center of mass at a place
1050 times farther away from Jupiter than from the Sun.  The distance between 
the center of Jupiter and the center of the Sun is 5.2 AU = 7.8 * 10^8 km; so
the center of mass is approximately

  (7.8 * 10^8 km) / (1050 + 1) = 7.4 * 10^5 km away from the Sun's center, 

directly between Jupiter and the Sun.  If Jupiter is on the far side of the Sun as 
seen by the distant astronomer, then the center of mass is 7.4 * 10^5 km farther 
away from the astronomer than the Sun's center.


(b) Using the same reasoning as Part (a): if Jupiter is on the near side of the 
Sun as seen by the distant astronomer, then the center of mass is 7.4 * 10^5 km 
closer to the astronomer than the Sun's center.

(c) The total distance would be (7.4 * 10^5 km) + (7.4 * 10^5 km) = 1.5 * 10^6 km.
We realize that during Jupiter's orbit around the Sun, its distance to the 
astronomer is always somewhere between Jupiter's "far side" and "near side" 
distance.