If the points with position vectors $60 \bar i+ 3 \bar j, 40 \bar i-80 \bar j$ and $a \bar i-52 \bar j$ are collinear, then $a = $

Question

\[\begin {array} {1 1} (1)\;-40 & \quad (2)\;-20 \\ (3)\;20 & \quad (4)\;40 \end {array}\]