There are several important information's that has already been provided in the question. Using these information's the answer to the given question can be easily found. Let us write down the given information's first.
Interest earned = $2800
Rate of Interest = 4%
Time for which the money is kept = 3.5 years
Let us assume the principal amount to be = x
Now we already know
Interest earned= Principal * Rate of Interest * Time
Principal = Interest earned/(Rate of Interest * Time)
= 2800/(4% * 3.5)
= (2800 * 100)/(4 * 3.5)
= 280000/14
= 20000
So the amount Andrea should deposit is $20000. This is the only method of solving these kind of problems very easily. Hopefully you have got the answer you were looking for.
