Name one chemical barrier the human body uses to defend itself against pathogens.

An investigator researched the antimicrobial activity of lysozyme, a chemical barrier found in human tears, against a species of bacterium.

The investigator used the following method:

Prepare four test tubes, each containing $cm315\text{ cm}^3$ of sterile nutrient broth.
Add lysozyme extract to adjust the concentrations to $0%0\%$ , $2%2\%$ , $4%4\%$ , and $8%8\%$ .
Add $cm31\text{ cm}^3$ of a liquid culture of the bacterium to each test tube.
Take a $cm30.1\text{ cm}^3$ sample from each test tube immediately (at 0 hours) and record the number of live bacterial colonies.
Incubate the test tubes at $37∘C37^\circ\text{C}$ for 12 hours.
Take another $cm30.1\text{ cm}^3$ sample from each test tube at 12 hours and record the number of live bacterial colonies.

Table 1 shows the results of this investigation.

Time in hours	Number of live bacteria at lysozyme concentration
	0%	2%	4%	8%
0	640	640	640	648
12	400	Y	80	18

Name one chemical barrier (other than lysozyme in tears) the human body uses to defend itself against pathogens.

What fraction of the bacteria present at 0 hours for 4% lysozyme concentration survived for 12 hours? Give your answer in its simplest form.

How many more bacteria were killed at 8% lysozyme concentration than at 0% lysozyme concentration in 12 hours? Complete the following steps:

A student calculated value Y in Table 1 to be 240. Suggest how the student calculated this value.