Find the area of these figures

The number of emergencies between 400 sec and 500 sec will be determined as follows:
z=(x-μ)/σ
but 
n=288
thus

when x=400
z=(400-450)/50=-1
P(z<-1)=0.4602

When x=500
z=(500-450)/50=1
P(z<1)=0.5398
Hence:
P(400<x<500)
=0.5398-0.4602
=0.0796
thus the number of cases will be:
0.0796×288
=22.9~23 emergency cases