Q9 of 110 Page 58

Suppose that five people are ill during the first week of an epidemic and each sick person

spreads the contagious disease to four other people by the end of the second week and so on. By the end of 15th week, how many people will be affected by the epidemic


People infected in first week = 5

More people infected by each person = 4


Number of weeks = 15


a = 5, r = 4, n = 15


Sum of n terms =


⇒ Sum of 15 terms =


⇒ Sum of 15 terms =


So, Number of people infected by 15th week =


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