A total of $7000 is invested: part at 7% and the remainder at 11%. How much is invested at each rate if the annual interest is $640?
a. $3,200 at 7%, $3,800 at 11%
b. $3,800 at 7%, $3,200 at 11%
c. $4,000 at 7%, $3,000 at 11%
d. $3,000 at 7%, $4,000 at 11%