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i've been having trouble understanding a part in this question. i'll just map out the basics to the question: 2 stars with a velocity of 220km/s are orbiting at equal distance. the masses of the stars are equal. period = 14.4 days it asks to find the mass of each star.

i understand that u can use kepler's law to get the answer.

GMm/r

^{2}= mv

^{2}/r

-which is what i used to come up with my answer seeing that i found the mass relative to a center mass. but according to the solution it's suppose to be:

GMm/(2r)

^{2}= mv

^{2}/r

my question is, why is r denoted 2r in order to arrive at a correct answer?