A $400,000 mortgage amortized over 25 years with monthly payments at an interest rate of 6% compounded semi-annually. Suppose you made a double up payment every six months (in other words every six months you make an extra payment) and at the end of each year you make a lump sum payment of $4000. How long would it now take to pay off the mortgage (calculate the number of years)? How much interest is paid over the life of the mortgage?