How many gorillas?
If the numbers in this news article have been estimated, then what is the largest number of gorillas that there could have been 10 years ago?
Age
14 to 16
Challenge level
Problem
A newspaper article about the wildlife in an African country contains the sentence "The number of gorillas has fallen by 70% in the last 10 years, and there are now only around 5000 left."
If 70% has been given to the nearest 1%, and the number of gorillas left now is correct to the nearest 1000, then what is the largest number of gorillas that there could have been 10 years ago?
This problem is adapted from the World Mathematics Championships
Student Solutions
Answer: 18640
Image
To maximise the number of gorillas we want the minimum value of 30% and the maximum value of 5000
ie. 29.5% and 5499
Unitary method
29.5% = 5499
1% = 5499$\div$29.5 = 186.4068
100% = 186.4068$\times$100 = 18640.68 so 18640 gorillas (cannot round up as this is maximum).
Multipler method
29.5% = 0.295, 0.295$x$ = 5499, so $x$ = 5499 $\div$ 0.295 = 18640.68 so 18640 gorillas (cannot round up as this is maximum).