We find the LCM of 18 and 21 Line up the prime factorizations, then bring them down to the bottom line and multiply: 18 = 2×3×3 21 = 3×7 ------------- LCM = 2×3×3×7 = 126 Answer = 126 minutes [which is 6 complete laps for the slower runner, since 6×21=126 and 7 complete laps for the faster runner, since 7×18 also = 126. Edwin