Suppose a bank can borrow at the monthly base rate im,b and lend out money at the sterling credit card lending rate im,c. Suppose a fraction d of borrowers never pay back the interest or what they borrow.

a). Derive a formula that calculates the maximum value of d under which the bank still makes a profit lending money and

b). calculate its average value for the sample period from your date of birth till the last month of available data. [10 percent]