4. You measure the luminosity of a newly discovered asteroid for 20 nights
in a row.  The results, rounded off to the nearest 5 units, are as follows:

   Day  1: 110 units   Day  2: 115 units  Day  3: 120 units  Day  4: 115 units
   Day  5: 110 units   Day  6: 105 units  Day  7: 100 units  Day  8: 105 units
   Day  9: 110 units   Day 10: 115 units  Day 11: 155 units  Day 12: 115 units
   Day 13: 110 units   Day 14: 105 units  Day 15: 100 units  Day 16:  95 units
   Day 17: 115 units   Day 18: 115 units  Day 19: 120 units  Day 20: 115 units

Based on these observations only, answer the following questions.
   (a) What is your best estimate of the luminosity of this asteroid?
   (b) What is the approximate margin of error for your measurements?
   (c) Does there appear to be a systematic effect that may affect the 
interpretation of your data?  If so, explain what it is; if not, explain why you 
think not.