One caution sign flashes every 4 seconds, and another caution sign flashes every 10 seconds. At a certain instant, the 2 signs flash at the same time. How many seconds elapse until the 2 signs next flash at the same time?
The correct answer is D.
The two signs will flash at the same time when the elapsed time is a common multiple of the signs’ flash intervals, 4 seconds and 10 seconds.
The least common multiple of 4 and 10 is 20, so the signs will flash at the same time after 20 seconds elapse.