Neither One nor Two but Three Geometric Series in One

Let $\frac{1}{7}$ + $\frac{3}{7^2}$ + $\frac{6}{7^3}$ + $\frac{10}{7^4}$ + $\frac{15}{7^5}$ + $...$ = $\frac{a}{b}$ , with $a$ and $b$ are coprime, what is $a + b$ ?