Question

A container contains water at temperature 80 degree celcius. After every 10 minutes, the change in temperature is - 4 degree celcius. After how much time will the temperature of the water be 20 degree celcius ?

Answer 1

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:
 $y = (-4/10) x + 80$
 For y = 20 we have:
 $20 = (-4/10) x + 80$
 Clearing x:
 $(4/10) x = 80 - 20 x = (10/4) * 60 x = 150$
 Answer:
 The temperature of the water will be 20 degree celcius after 150 minutes