At clock cycle=8, $I_{6}$'s execute instruction will end and hence at clock cycle=9, $I_{12}$'s Stage 1 will start and $I_{12}$ will complete at clock cycle=12. Now the remaining $I_{13}$ to $I_{16}$ instructions will complete at clock cycle=16. Since, cycle time of pipeline is 4 ns, hence Time Required =
4ns * 16 = 64 ns