1. Let's first state the problem: We want to find the exact thickness of lithium when it is discharged, given the thickness when fully charged and the thickness when discharged. 2. The key formula to use here is: $$\text{Thickness}_{\text{discharged}} = \text{Thickness}_{\text{charged}} - \text{Change in thickness}$$ 3. Important rule: The "change in thickness" is the difference between the fully charged thickness and the discharged thickness. If you know the fully charged thickness and the thickness after discharge, subtracting the discharged thickness from the charged thickness gives the change, but to find the discharged thickness from the charged thickness and change, you subtract the change from the charged thickness. 4. If you have the fully charged thickness $T_c$ and the thickness when discharged $T_d$, then the change in thickness $\Delta T$ is: $$\Delta T = T_c - T_d$$ 5. To find the exact thickness of lithium when discharged, you do: $$T_d = T_c - \Delta T$$ 6. So yes, you subtract the change in thickness (which is the difference between fully charged and discharged thickness) from the fully charged thickness to get the exact thickness when discharged. 7. In summary, the exact thickness of lithium when discharged is the fully charged thickness minus the thickness difference caused by discharge.