Question

Three red lights on the same road start blinking at the same time and blink repeatedly after every 4, 6 and 8 minutes, respectively. How many times will they blink together in an interval of 3 hours, excluding blinking together at the starting moment?

• A. 5
• B. 7
• C. 6
• D. 4

Answer 1

Answer:

B. 7

They blink together at intervals equal to the LCM of 4, 6, and 8 minutes.

LCM(4, 6, 8):
4 = 2², 6 = 2 × 3, 8 = 2³
LCM = 2³ × 3 = 24 minutes

So, they blink together every 24 minutes.

Total time = 3 hours = 180 minutes

Number of times they blink together =
$\frac{180}{24} = 7.5$

So they blink together 7 times after the start (since we exclude the starting moment).