A person purchased a $1,145,000 home 10 years ago by paying 25% down and signing a 30-year fixed rate mortgage at 7.8% p.a.. Interest rate have dropped and the owner wants to refinance the unpaid balance by signing a new mortgage fixed 4% p.a.. Assume monthly mortgage payment frequency.

1. Determine the amount of monthly payments under the original mortgage contract without refinancing.
2. After how long will the principal outstanding be reduced by a half?
3. How much interest will refinancing save?

Can i have the whole working steps? Thank you soooo mcuh!