  The coordination among drones and ground vehicles for last-mile delivery has
gained significant interest in recent years. In this paper, we study
\textit{multiple drone delivery scheduling problem(MDSP) \cite{Betti_ICDCN22}
for last-mile delivery, where we have a set of drones with an identical battery
budget and a set of delivery locations, along with reward or profit for
delivery, cost and delivery time intervals. The objective of the MDSP is to
find a collection of conflict-free schedules for each drone such that the total
profit for delivery is maximum subject to the battery constraint of the drones.
Here we propose a fully polynomial time approximation scheme (FPTAS) for the
single drone delivery scheduling problem (SDSP) and a
$\frac{1}{4}$-approximation algorithm for MDSP with a constraint on the number
of drones.
