Amodel hydroelectric power station produces just enough electric power to light a 6.0 w lamp. the model is found to be 80% efficient at converting the potential energy store of the water into electrical work. what is the input power of the water running through the pipes?

You said that the useful output power from the model is 6 watts.  You also said that the model is 80% efficient.  We have no reason to doubt your word, so we know that

(useful output) / (input power) = 80% or 0.8

(6 watts) / (input power) = 0.8

Multiply each side by (input power): 6 watts = (0.8) (input power)

Divide each side by 0.8 :  Input power = (6 watts/ 0.8)

Input power =  7.5 watts

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me