My Question:

Consider the mortgage scenario in Worksheet $1.$ If the monthly mortgage installment is $2$C instead and the term of the mortgage remains unchanged, approximate the new effective interest rate using linear interpolation between $9\%$ and $10\%$ per annum. Show all steps.

Worksheet $1$:

A company purchases a residential unit for $$7\mathrm{.}5$ million, with $60\%$ of the sum financed by a $30-$year mortgage with equal monthly installments. Suppose the prevailing interest rate is fixed at $3\%$ per annum effective $^(1).($a$)$ Calculate the monthly mortgage installment, C$.($b$)$ An installment can be decomposed into two components: payment of interest and repayment of principal. From which point onwards will the repayment of principal exceed the payment of interest?

Answer from Worksheet $1$: C$=18874\mathrm{.}089$