If it's a simple interest = $38.4
If it's a compound interest = $678.98
Solutions
Simple interest, I = principal x rate x time
Principal = $640
Time = 2 years
Rate = 3% = 3/100 = 0.03, then substitute respectively.
Compound interest, A = P (1 + r)^t
Substitute respectively. Remember, always divide your rate in percent by 100.
Don't forget to read the question and approximate correctly.