The number of days will be a multiple of both 4 and 5. The smallest such multiple is called the "least common multiple" (LCM). One way to figure the LCM is to divide the product of the numbers by their greatest common factor. Since 4 and 5 have no common factors, the LCM is 4·5 = 20.
Both teams will be practicing on the same day again in 20 days.
