Simplify the following text for a Grade 7 native English-speaking person.
Original: Likewise , the distance traveled by light during 1 Planck time is 1 Planck length . To determine , in terms of SI or other existing system of units , the quantitative values of the Planck units , those equations and three others must be satisfied to determine the five unknown quantities that define the Planck units :
Simplified: Planck units are only one system of natural units among other systems .