Cost = (0.001) x (the wattage of the light) x (the number of hours it's left on) x (the cost of each kilowatt-hour of electrical energy where you live).
To develop this problem it is necessary to apply the concepts related to Sound Intensity.
By definition the intensity is given by the equation

Where,
I = Intensity of Sound
= Intensity of Reference
At this case we have that 15 engines produces 15 times the reference intensity, that is

And the total mutual intensity is 100 dB, so we should




Therefore each one of these engines produce D. 88dB.
Light waves can be from any color, depends on what it is bouncing no or reflecting off of.
It makes it easier to dig a hole because it helps pick up dirt