Question

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 15^th week, how many people will be affected by the epidemic?

Answer 1

Number of people affected by the epidemic during each week form a geometric series. 

S₁₅ = 5 + (4 x 5) + (4 x 20) + (4 x 80) + … 15 terms 

= 5 + 20 + 80 + 320 + … 15 terms 

Here a = 5, r = 4, n = 15