Independent solution

How to solve this Force of Interest question

Setup

Setup

Measure the requested calendar year from the stated January 2020 origin.

tstart=4,tend=5t_{\mathrm{start}}=4,\qquad t_{\mathrm{end}}=5

Model

Model

An accumulation factor under a time-varying force is the exponential of its integral.

1+i=exp(45dt20+t)1+i=\exp\left(\int_4^5\frac{dt}{20+t}\right)

Compute

Compute

Integrate the logarithmic derivative and exponentiate.

45dt20+t=ln(2524)\int_4^5\frac{dt}{20+t}=\ln\left(\frac{25}{24}\right)
i=25241=0.0416667i=\frac{25}{24}-1=0.0416667

Answer

Answer

The equivalent annual effective rate is 4.17%, choice C.

i4.17%(C)\boxed{i\approx4.17\%\quad\text{(C)}}