James’s four-year-old brother is trying to arrange black and yellow legos in rows. he has 56 yellow legos and 120 black legos. he wants to arrange them in rows so that there are either only yellow or only black legos in each row, and so that each row has the same number of legos. what is the least number of rows he can make?

To calculate the size that the rows should have, perform a decomposition in the prime factors of each number.
 For 120 its decomposition is: 5 x 3 x (2 ^ 3)
 For 56 its decomposition is: (2 ^ 3) x 7
 So we have that the common factor is 2 ^ 3 = 8.
 Therefore, the rows must be 8 laymen.
 The number of rows for yellow legos is: 7
 The number of rows for blacks is: 5 x 3 = 15
 In summary, you must make 7 rows of 8 yellow legos and 15 rows of 8 black legos