If the locus of $z\in\mathbb{C}$, such that $\operatorname{Re}!\left(\dfrac{z-1}{2z+i}\right)+\operatorname{Re}!\left(\dfrac{z-1}{2z-i}\right)=2$, is a circle of radius $r$ and center $(a,b)$, then $\dfrac{15ab}{r^2}$ is equal to: