IAS 2023 — CSAT Maths

Correct Option (B)

To determine when the three traffic signals will next turn green simultaneously, it is necessary to calculate the Least Common Multiple (LCM) of their respective full cycle durations. The problem states that each signal takes a specific time to change from green to red, and the durations for both green and red colours are equal. Therefore, the full cycle duration for each signal (from green, through red, and back to green) is twice the given time:

• Signal 1 cycle duration: 25 seconds (green) + 25 seconds (red) = 50 seconds.
• Signal 2 cycle duration: 39 seconds (green) + 39 seconds (red) = 78 seconds.
• Signal 3 cycle duration: 60 seconds (green) + 60 seconds (red) = 120 seconds.

Next, calculate the LCM of these full cycle durations (50, 78, and 120):

• Prime factorization:
• $50 = 2 \times 5^2$50=2×52
• $78 = 2 \times 3 \times 13$78=2×3×13
• $120 = 2^3 \times 3 \times 5$120=23×3×5
• $LCM(50, 78, 120) = 2^3 \times 3 \times 5^2 \times 13 = 8 \times 3 \times 25 \times 13 = 7800$LCM(50,78,120)=23×3×52×13=8×3×25×13=7800 seconds.

Convert the total time from seconds to minutes and hours:

• $7800 \text{ seconds} = \frac{7800}{60} \text{ minutes} = 130 \text{ minutes}$7800 seconds=607800​ minutes=130 minutes.
• $130 \text{ minutes} = 2 \text{ hours and } 10 \text{ minutes}$130 minutes=2 hours and 10 minutes.

The signals initially turn green together at 2:00 p.m. Adding the calculated duration:

• $2:00 \text{ p.m.} + 2 \text{ hours } 10 \text{ minutes} = 4:10 \text{ p.m.}$2:00 p.m.+2 hours 10 minutes=4:10 p.m..

Thus, the signals will next change to green simultaneously at 4:10 p.m.

Incorrect Options:

Options (A) 4:00 p.m., (C) 4:20 p.m., and (D) 4:30 p.m. do not correspond to the Least Common Multiple of the full cycle durations of the three traffic signals. These times would not result in all signals turning green simultaneously after the initial 2:00 p.m. instance, as they do not align with the calculated 2 hours and 10 minutes interval.