Step 1 :\(\text{Amount to finance} = 330,500 \times 0.80 = 264,400\)
Step 2 :\(\text{For the 15 year mortgage at 3.5%:}\)
Step 3 :\(r = \frac{3.5}{12} = 0.002917\)
Step 4 :\(n = 15 \times 12 = 180\)
Step 5 :\(P = \frac{0.002917 \times 264,400}{1 - (1 + 0.002917)^{-180}} = 1,889.70\)
Step 6 :\(\text{For the 20 year mortgage at 7.0%:}\)
Step 7 :\(r = \frac{7.0}{12} = 0.005833\)
Step 8 :\(n = 20 \times 12 = 240\)
Step 9 :\(P = \frac{0.005833 \times 264,400}{1 - (1 + 0.005833)^{-240}} = 2,056.61\)
Step 10 :\(\text{For the 15 year mortgage:}\)
Step 11 :\(\text{Total payments} = 1,889.70 \times 180 = 340,146\)
Step 12 :\(\text{Total interest} = 340,146 - 264,400 = 75,746\)
Step 13 :\(\text{For the 20 year mortgage:}\)
Step 14 :\(\text{Total payments} = 2,056.61 \times 240 = 493,586.40\)
Step 15 :\(\text{Total interest} = 493,586.40 - 264,400 = 229,186.40\)
Step 16 :\(\text{Difference in interest paid} = 229,186.40 - 75,746 = \boxed{153,440.40}\)