I used Pythagorean theorem to solve this problem. As DC and AD is 12 and 5, you would do 12²+5²= c². 12²= 144 and 5²= 25. 144+25= 169. It doesn't end there. Do the square root of 169 ---> √169=13.
There could be problems like there is a light which would flicker in every 7 seconds and there is another one which would flicker in every 8 seconds. So, how many times would they flicker until they would flicker at the same time?