If a planet takes 100 years to revolve around the sun, how long is their day? Is there information missing?
The time taken by a planet to revolve around the Sun is how long a year on the planet is. In this case one year on the planet has the same duration as 100 Earth years. It is not possible to determine how long a day on the planet is given only the time it takes to revolve around the Sun as the two are not correlated. For example, a year on Venus is equal to 224.68 Earth days but a day on Venus is equal to 243 Earth days; on the other hand a year on Neptune is equal to 164.81 Earth years but a day is only 19.1 Earth hours.
It is not possible to determine how long a day on the planet is with the information provided.
The missing information is the time taken by the planet to rotate on its axis.