(8400)(0.15) = 1260
8400-1260 = 7140
by taking their lowest common multiple, we get an answer of 30. in which in every 30 seconds, all of them will blink at the same time.
now every minute there is 60 seconds, dividing our previous answer which is 30, we get an answer of 2 blinks per minute. multiplying that to 60 minutes, we get a total of 120 blinks all in all.
just subtract the sum of the 2 sides from the perimeter, to get the 3rd side