Final answer:
The ratio of the number of output coils to the number of input coils in a step-up transformer to increase the voltage fivefold is 20 times.
Step-by-step explanation:
The ratio of the number of output coils to the number of input coils in a step-up transformer to increase the voltage fivefold is 20 times.
A step-up transformer is used to increase voltage. The ratio of the number of output coils to the number of input coils is equal to the voltage ratio. In this case, the voltage is being increased fivefold, so the ratio is 5:1. Since the question asks for the ratio, the answer is 20 times.