help needed with this question.

48. The United States Census Clock has ashing light signs to indicate gains and losses in the popula- tion. Here are the time periods of these ashes in seconds: Birth, 10; Death, 16; Immigrant, 81; Emigrant, 900. In other words, every 10 seconds there is a birth, every 16 seconds a death, and so on. Assume that these lights all started ashing at the same moment. a. If you saw the birth and emigrant signs light up at the same time, how many seconds would pass before they would both light together again? Suppose you saw the immigrant and emigrant signs both light at the same time. What is the shortest time before they will both be lit together again? It is possible for the birth and death signs to light together every 80 seconds. Why? What is the increase in population during a one-hour period