Once way you can do this is to set a table:

[HTML]<table border = ".1" cellpadding = "4">

<tr>

<td> </td><td> Deposited</td><td>Interest Earned</td><td>Total Balance</td>

</tr>

<tr>

<td>End of year 1</td><td>400</td><td>0</td><td>400</td>

</tr>

<tr>

<td>End of year 2</td><td>400</td><td>.04*400 = 16</td><td>400+400+16 = 816</td>

</tr>

<tr>

<td>End of year 3</td><td>400</td><td>.04*816 = 32.64</td><td>816+400+32.64 = 1248.64</td>

</tr>

</table>[/HTML]

and continue in this fashion for 12 years. Or observe that:

after 1 year you have a balance of ,

after 2 years you have a balance of

after 3 years your balance is

and after n years you have a balance of 400((1.04)^0 + (1.04)^1 + ... + (1.04)^{n-1})

The sum of powers of is a geometric series which simplifies to the formula:

So after years you should have saved: .