Billy contracted a loan. He pays back his loan in 3 payments : the first one is $7000 made after one year, the second one of $5000 made one year and a half after he contracted the loan, and the last one, of $3000 , made two years after he contracted the loan. The interest rate is i^(2) = 10% .

(a) Determine the amount of the loan.

(b) If after his first payment, Billy renegotiates his loan with the bank such that he only has one payment left of $ X to do, four years after he contracted the loan, with the same interest rate, what should be the value of X ?