Independent solution

How to solve this Interest Credited over a Specified Year question

Setup

Setup

Integrating the force gives an accumulation proportional to t plus 1. The initial deposit of 3 therefore has value 3(t+1).

A(t)=3exp(0tdss+1)=3(t+1)A(t)=3\exp\left(\int_0^t\frac{ds}{s+1}\right)=3(t+1)

Model

Model

Interest earned during the third year is the value increase from time 2 to time 3.

A(3)A(2)=129=3A(3)-A(2)=12-9=3

Compute

Compute

The bank account's third-year interest is the difference of its time-3 and time-2 values.

X(1.0531.052)=0.055125XX(1.05^3-1.05^2)=0.055125X
X=30.055125=54.4218X=\frac3{0.055125}=54.4218

Answer

Answer

The required bank deposit is 54.4, choice E.

X54.4(E)\boxed{X\approx54.4\quad\text{(E)}}