Given, Five traffic lights on simultaneously at 1 pm Formula: The least number which is exactly divisible by each one of the given number is called their LCM. Calculation:12=2×2×3 15=3×5 45=3×3×5 60=2×2×3×5 90=2×3×3×5 LCM of 12, 15, 45, 60 and 90 = 2 × 2 × 3 × 3 × 5 = 180 min = 3 hrs ∴ All five traffic lights on next time = 1 pm + 3 hrs = 4 pm.