A student investigates how wrapping different numbers of layers of bubble wrap around a metal cup containing hot water changes its cooling rate.

The student writes this plan for her investigation:

I will use six identical metal cups.
I will wrap each metal cup with a different number of layers of bubble wrap (from 0 to 5).
I will pour 200 cm³ of hot water into each cup and wait until the temperature of the water cools to $80∘C80^\circ\text{C}$ .
I will then start a timer and record the final temperature of the water after $minutes10\text{ minutes}$ .

State the independent variable in the student's investigation.

State the dependent variable in the student's investigation.

The table below shows the student's results:

Number of layers of bubble wrap	Final temperature ( $∘C^\circ\text{C}$ )	Temperature decrease ( $∘C^\circ\text{C}$ )
0	50	30
1		24
2		20
3		18
4		18
5		18

Complete the table by calculating the missing final temperatures.

Describe the relationship between the number of layers of bubble wrap and the temperature decrease of the water.

Suggest how the student could improve the reliability of their results.