Let us say, the CD amount she invested is "c"
and the amount she invested in the savings bond is "s"
she invested a total of 5,000
so, whatever "c" is and "s" is, c + s = 5,000
now, the CD account earned 4% in interest
4% in decimal form is 4/100 or 0.04
she also earns 7% from the saving bonds,
7/100 is 0.07
the interest earned after one year are 300
that is, the 4% per year and the 7% per year add up to 300 bucks
so, one can say that
0.04c + 0.07s = 300