The spider is traveling in a circle with radius = 15cm
The circumference of any circle = 2 pi (radius)
The circumference of the spider's path = 2 pi (15 cm) = 30 pi cm
The spider completes a trip around this path 78 times per minute.
Its speed, relative to you, is
(78) x (30 pi) cm/min =
2,340 pi cm/min = 7,351.33 cm/min =
73.5133 meter/min =
4.411 km/hr =
2.74 miles/hour
(After the last appearance of pi,
all numbers are rounded.)