Assuming your retirement is more than 2 years away, and that your car purchase is not before the end of the year, the shortest-term goal of those listed is ...
... d. add $200 to your savings account by the end of the year
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As a teenager, "short term" may be a matter of a week or two. As a middle-aged adult, both your life-lived and life-expectancy are somwhat longer, so "short-term" can be somewhat longer as well. In old age and failing health, "short-term" may again be a matter of a few weeks at most. Such concepts are tempered by perspective.
Answer:
Binary systems are the easiest, where there are two stars in a solar system instead of one.
Explanation:
When there are two stars in a solar system, instead of one, the stars orbit each other. Astronomers can measure quite a bit of information about the movement of the stars. This includes how fast they travel around each other and how often they fully orbit each other. From this information, the mass can be calculated. Note that this information can then be used to estimate the masses of other stars that fit the same type of categories in solar systems with a single star.