If 1 / a plus 1 / b equals 1 / 3, 1 / a plus 1 / B plus 1 / C equals 5 / 7, then what is C equal to?

1/a+1/b=1/3
1/a+1/b+1/c=5/7
1/c=5/7-1/3=15/21-7/21=8/21
c=21/8

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