Acontainer 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 ?

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:
 
 For y = 20 we have:
 
 Clearing x:
 
 
 The temperature of the water will be 20 degree celcius after 150 minutes