# What kind of a sum is this?

Algebra Level 4

When the value of the sum $$\displaystyle\sum_{n=1}^{\infty} \biggl( n \cdot \dfrac{3^{n-1} \cdot 2}{5^n}\biggr)$$ is expressed in the form $$\dfrac{a}{b}$$ for relatively prime positive integers $$a$$ and $$b$$, what is the value of $$a+b$$?

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