Question 631622
This is and arithmetic progression

Starting a= 10
next month it is 15 so the difference d = 5
the number of months = n = 7
Apply the formula
Tn=a+(n-1)d where Tn is the nth. term

T7 = 10 +(7-1)*5

T7 = 10+30

T7 = 40 

Population at the end of 7 months = 40

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