Delhi Police Constable Exam Model Paper 10

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: $12=2\times 2\times 3$
$15=3\times 5$
$45=3\times 3\times 5$
$60=2\times 2\times 3\times 5$
$90=2\times 3\times 3\times 5$
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.