The traffic lights at three different road crossings change after every 90 seconds, 120 seconds and 150 seconds respectively. If they all change simultaneously at 5:20 p.m., then find the time (in p.m.) at which they will change simultaneously again?
Show answer & explanation
Correct answer: D
Explanation
90 = 2 × 32 × 5
120 = 23 × 3 × 5
150 = 2 × 3 × 52
Least Common Multiple (LCM) = 23 × 32 × 52
= 1800 seconds
= 30 minutes
5:20 PM + 30 minutes = 5:50 PM