For this case, the first thing we must do is define variables.
We have then:
x: number of minutes
y: final temperature
We write then the equation that models the problem:
[tex]y = (-4/10) x + 80
[/tex]
For y = 20 we have:
[tex]20 = (-4/10) x + 80
[/tex]
Clearing x:
[tex](4/10) x = 80 - 20
x = (10/4) * 60
x = 150[/tex]
Answer:
The temperature of the water will be 20 degree celcius after 150 minutes
