Since a temperature was not provided, I chose a random number: 5. I then increased it by 5% & then again by 20%. Then compared the resulting temperature to the original temperature to see how much it had increased by overall.
5×.05=.25
5+.25=5.25
5.25×.20=1.05
5.25+1.05=6.3
5/6.3=.794
The original temperature (5) was 79.4% of the temperature after increase (6.3). Meaning it raised by 20.6%
(100-79.4=20.6)
I'm not sure if this is how your teacher would like you to solve it, but this is how I solved it.
The first (80%) mortgage has an interest rate of 4.75%, and the second (20%) mortgage has an interest rate of 7.525%. Both are 30-year fixed-rate mortgages.