In FCFS scheduling, processes execute in the same order in which they arrive.

Order is:

$P1 \to P2 \to P3 \to P4$

Waiting time of $P1$:

$0$

Waiting time of $P2$:

$8$

Waiting time of $P3$:

$8+15=23$

Waiting time of $P4$:

$8+15+10=33$

Total waiting time:

$0+8+23+33=64$

Average waiting time:

$\frac{64}{4}=16$