"What is the interest rate if borrowing equals saving?" means that at what interest rate for a future return that makes you want to save money today while at this interest rate this amount of saving allows others to borrow for the current-period consumption, e.g., for a borrower borrowing is $833 in period 1 and paying back is $1,000 in period 2 (equivalently saying, saving is $833 for a saver today and getting back is $1,000 in the future.), so what is r such that makes it occur by solving this equation: $833*(1+r) = $1000? (This makes sense once we draw the diagrams for both the saver and borrower with all points marked, and how did we know there will be $1,000 paying back[getting back] from $833? We look at a loss [and a gain] of $1,000 in future income of $10,000 (endowment) for a borrower [and for saver] as we can observe it from the vertical axis C2 on the graphs.)

To receive full credits, you are required to show your work and demonstrate your explanation and knowledge of the graphical analysis of the two effects (S.E. & I.E.) for your response answer, e.g., there should not be showing a simple replica of a diagram for your response without short but concise explanation.