t % 5

% modulus operator return remainder 5%2 = 1 3%2 = 1 12 %2 = 0 and 1 hours = 60 minutes as the condition says the traffic light works on hourly basis that will be GREEN for first 3 minutes YELLOW for next 1 minute. and RED for next 1 minute. This cycle will move on up to 60 minutes. here t = time of the task that is from (1 to 60 minutes) . Now moving to Second part of your question why this seems to then mean the higher numbers are then covered? Before answering that Try one thing yourself as I suggest try to find out modulus of each minute from (1to 60) minutes by 5 as follows 1%5 2%5 3%5 4%5 5%5 .........and so on move upto 55%5, 56%5,....60%5 and observe the output. See if you can get the answer of your second part. Just Try at least once.