In the image, you are asked to calculate the $R_f$ value of substance A using the following formula:

$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$

From the details provided:

• Distance moved by substance A = 60 mm
• Distance moved by solvent = 80 mm

Now, let's calculate the $R_f$ value:

$R_f = \frac{60 \, \text{mm}}{80 \, \text{mm}} = 0.75$

Therefore, the $R_f$ value of substance A is 0.75.

Would you like further details or explanations about this concept?

Here are some related questions for further learning:

1. What does the $R_f$ value represent in chromatography?
2. How can different substances be separated using chromatography?
3. What factors can affect the $R_f$ value of a substance?
4. What are some common applications of chromatography in scientific research?
5. How does the polarity of the solvent influence the $R_f$ value?

Tip: Always ensure that the units for the distances in the $R_f$ calculation are the same (in this case, both in mm), so the ratio is meaningful.