The Student Room Group

Percentage increase and decrease

Q. The population of a town in the year 1990 was 15600.
The population of 15600 was 2.5% less than the population in the year 1980.
Calculate the population in the year 1980.
//The answer is 16000 but when I did it I got 15990.
I did (15600*102.5)/100
The right method is 15600 * (100/100-2.5)
CAN someone tell me why my method is wrong//
Original post by 123IGSTUDENT
Q. The population of a town in the year 1990 was 15600.
The population of 15600 was 2.5% less than the population in the year 1980.
Calculate the population in the year 1980.
//The answer is 16000 but when I did it I got 15990.
I did (15600*102.5)/100
The right method is 15600 * (100/100-2.5)
CAN someone tell me why my method is wrong//


Your method takes 2.5% of 15600 not the original population in 1980.
To revert back to an original population divide by (100-percentage change) then times that answer by 100

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