See the attachment.
AO = height of street light = H
AB = distance of person from base of street light = 40 feet
AB = 40*12 inches = 576 inches
BC = shadow length = 96 inches
ΔOAC and ΔMBC are similar. So
483 inch = 483/12 feet = 40 feet 3 inch
So the street light is 40 feet 3 inch
above the ground.