The graph shows the temperature of water in a pot from 1500 to 15 30.

1. For how many minutes was the temperature of the water 70°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 1520 to 1525? Give your answer in °C/min.

(a)
Temperature of water that reached 50°C at 15 10.

From 15 05 to 15 30: 25 min

Duration that the temperature of water that reached 70°C and below = 25 min

(b)
Decrease in temperature of water
From 15 00 to 15 05: 87°C - 70°C = 17°C
From 15 5 to 15 10: 70°C - 50°C = 20°C
From 15 10 to 15 15: 50°C - 40°C = 10°C 
From 15 15 to 15 20: 40°C - 35°C = 5°C 
From 15 20 to 15 25: 35°C - 24°C = 11°C
From 15 25 to 15 30: 24°C - 13°C = 11°C

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

(c)
Change in temperature per minute from 15 20 to 15 25
= 11 ÷ 5
= 2.2°C/min

Answer(s): (a) 25 min; (b) 15 15 to 15 20, 15 20 to 15 25; (c) 2.2°C/min