Bunuel
The population of Town X on January 1, 2010, was 56 percent greater than the population of the same town on January 1, 2005. The population of Town X on January 1, 2015, was 75 percent greater than the population of the same town on January 1, 2010. What was the population of Town X on January 1, 2005?
(1) The population of Town X on January 1, 2015, was 21,840.
(2) The increase in the population of Town X from January 1, 2010, to January 1, 2015, was 4,880 greater than the increase in the population of Town X from January 1, 2005, to January 1, 2010.
IMO D,
Assuming population of village X at the start of Jan 1, 2005 as 100x.
Jan 2005 Jan 2010(56% greater than 2005) Jan 2015(75% greater than 2010)
100x 100x + 56% of 100x = 156x 156x + 75% of 156x = 273x.
Now,
1st - The population of Town X on January 1, 2015, was 21,840.
273x = 21840 after solving we will get x = 80
So, the population of Town X on January 1, 2005 = 100x = 80*100 = 8000. Hence Sufficient.
2nd - The increase in the population of Town X from January 1, 2010, to January 1, 2015, was 4,880 greater than the increase in the population of Town X from January 1, 2005, to January 1, 2010.
Increase in population from jan1, 2010 to jan1, 2015 is 75% of 156x which is equal to 117x
increase in population from jan1, 2005 to jan1, 2010 is 56% of 100x which is equal to 56x.
forming equation as per given statement,
117x = 4880 + 56x
117x - 56x = 4880
61x = 4880
x=80
So, the population of Town X on January 1, 2005 = 100x = 80*100 = 8000. Hence Sufficient.
Both alone sufficient Hence D.