The graph shows the temperature of water in a pot from 2100 to 21 30.

1. For how many minutes was the temperature of the water 80°C and below? Give your answer in min.
2. During which two 5-minute intervals did the temperature of water decrease at the same rate? Give your answer in the following format. (Eg 01 00 to 01 15, 01 25 to 01 30)
3. What was the change in temperature per minute from 2120 to 2125? Give your answer in °C/min.

(a)
Temperature of water that reached 60°C at 21 10.

From 21 05 to 21 30: 25 min

Duration that the temperature of water that reached 80°C and below = 25 min

(b)
Decrease in temperature of water
From 21 00 to 21 05: 94°C - 80°C = 14°C
From 21 5 to 21 10: 80°C - 60°C = 20°C
From 21 10 to 21 15: 60°C - 43°C = 17°C 
From 21 15 to 21 20: 43°C - 37°C = 6°C 
From 21 20 to 21 25: 37°C - 22°C = 15°C
From 21 25 to 21 30: 22°C - 7°C = 15°C

Two 5-minute intervals that the temperature of water decreases at the same rate are 21 15 to 21 20 and 21 20 to 21 25.

(c)
Change in temperature per minute from 21 20 to 21 25
= 15 ÷ 5
= 3°C/min

Answer(s): (a) 25 min; (b) 21 15 to 21 20, 21 20 to 21 25; (c) 3°C/min