Question

The number of fish in a lake decreases by x% each year.

Given that the number of fish halves in 8 years, work out the value of x.
Give your answer correct to 1 decimal place.

Answer 1

Answer: Hence, the value of x is 8.3%.

Explanation:

Since we have given that

The number of fish in a lake decreases by x% each year.

Given : The number of fish halves in 8 years.

Let N be the number after 8 years.

So, our equation becomes:

`(N)/(2)=N(1-(x)/(100))^8\\\\(1)/(2)=(1-0.01x)^8\\\\0.5=(1-0.01x)^8\\\\\sqrt[8]{0.5}=1-0.01x\\\\0.917=1-0.01x\\\\0.917-1=-0.01x\\\\-0.0829=-0.01x\\\\(0.0829)/(0.01)=x\\\\8.3\%=x`

Hence, the value of x is 8.3%.

Answer 2

The number of fish decreases by x% each year. x% can also be written as 0.01x.

If the total number of fish in a lake is A, after one year the number of fish will be:


`A_(1)=A(1-0.01x)`

After two years the number of fish will be:


`A_(2)=A(1-0.01x)^(2)`

So, the general formula for the number of fish after n years can be written as:


`A_(n)=A(1-0.01x)^(n)`

It is given that after 8 years, the number of fish is reduced to half. So we can write:


`0.5A=A(1-0.01x)^(8) \\ \\ 0.5=(1-0.01x)^(8) \\ \\ 0.917=1-0.01x \\ \\ 0.01x=0.083 \\ \\ x=8.3`

This means the value of x is 8.3%. So, the number of fish in a lake decreases by 8.3% each year