First we will find the fixed monthly payment for each and then the total interest.

$110,000 mortgage: a 30-year fixed-rate at 8.5%

c = P0((r/12)/(1 - (1 + r/12)^(-N)), where P0 is the principal, r is the fixed rate as a decimal, N is the number of payments

c = 110,000*(0.085/12)/(1 - (1 + 0.085/12)^(-30*12))

c = $845.80

click here to see the step by step calculation:

Monthly payment: $845.80

Total interest: 12*30*845.80 - 110,000 = $194,488.00

$110,000 mortgage: a 15-year fixed-rate at 8%

c = 110,000*(0.08/12)/(1 - (1 + 0.08/12)^(-15*12))

c = $1,051.22

click here to see the step by step calculation:

Monthly payment: $1,051.22

Total interest: 12*15*1,051.22 - 110,000 = $79,219.60

More economical is $110,000 mortgage: a 15-year fixed-rate at 8%.

There is $194,488.00 - $79,219.60 = $115,268.4 saved in interest.