A city's population grows by 10% in the first year, 20% in the second year and 10% in the third year. If the population after 3 years is 8,71,200, then what was the original population 3 years ago?

Let the original population 3 years ago be P.
The population grows successively by 10%, 20%, and 10% over three years.

Final Population=P×(1+
100
10
​
)×(1+
100
20
​
)×(1+
100
10
​
)
871200=P×
100
110
​
×
100
120
​
×
100
110
​

871200=P×
10
11
​
×
5
6
​
×
10
11
​

871200=P×
500
726
​

P=
726
871200×500
​

Let's divide 871200 by 726:

726
871200
​
=1200
P=1200×500=6,00,000