At the end of each quarter, John deposits all his quarterly earnings into his savings account. The bank offers $12\%$ per half year with quarterly compounding. After $10$ years, John withdraws all the money he has accumulated in his savings account and uses it to provide a $5-$year loan with a fixed instalment of $12000$ EUR, repayable twice a year at the end of each period, and an interest rate of $16\%$ per annum with semi$-$annual compounding.

a$)$ Please calculate the principal $($amount$)$ of such a loan.

b$)$ Please calculate John's quarterly salary.

c$)$ Please calculate an effective interest rate equivalent to the interest rate offered on the savings account.

d$)$ What would be the answer to $($a$)$ if the loan is a perpetual loan $($ perpetuity due$).$

all other conditions remaining the same?