When I was in my 20s it would take me 20 hours work to earn 100 units

15 years later it takes 3 hours work to earn the same amount

If I borrow 100 units in my 20s at 4%, it cost 4 units per year

I save 20 hours work, and it costs me 4 units a year. That's 48 minutes work in year one, reducing to 8 minute after 15 years. Overall it costs an average 20 minutes a year for 15 years, or 5 hours of work, then I repay the capital which is 3 hours.

I've spent 8 hours of work rather than 20 hours.