$15a = \frac{1}{2} $
$a = \frac{1}{30}$
$3a + 3b = \frac{1}{3} $
$\frac{3}{30}+3b = \frac{1}{3}$
$b = \frac{7}{90}$
$2b + 2c = \frac{1}{3}$
$\frac{14}{90}+2c = \frac{1}{3}$
$c = \frac{8}{90}$
$a + b + c = \frac{1}{30} + \frac{7}{90} + \frac{8}{90} = \frac{1}{5} $
$\therefore \frac{3}{a+b+c} = 3 \div \frac{1}{5} = 15$
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