There is a string of red and blue lights strung around a window. The red lights blink every 3 seconds , and the blue lights blink every 5 seconds. How many times will they blink at the same time in one minute??Please show work

Well you could count by threes and Fives and see your answer, 3,6,9,12,15, and so on and fives 5,10,15 and you have one match. 18, 21, 24, 27, 30 and 20, 25, 30. Two matches. They will blink at the same time about six times.