Question 10 Exercise 4.2
Solutions of Question 10 of Exercise 4.2 of Unit 04: Sequence and Series. This is unit of A Textbook of Mathematics for Grade XI is published by Khyber Pakhtunkhwa Textbook Board (KPTB or KPTBB) Peshawar, Pakistan.
Question 10
The population of a town is decreasing by $500$ inhabitants each year If its population in 1960 was 20135, what was its population in 1970? 
Solution
Suppose $a_1$ represents the population in 1960. Then $$a_1=20135.$$ Decrease in population each year=$d=-500$.
The given problem is of A.P and we have to find population in 1970, that is, $a_{11}$.
As we know
\begin{align}
a_{11}&=a_1+10d \\
&=20135+10(-500)\\
&=15135. \end{align}
Hence the population of the town in $1070$ was $15135$.
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