B = amount invested in B bonds
c = amount invested in CDs
so.. she's got $70000 burning a hole in her pocket and she wants to invest them to get a yield of 5000 in interest from both of them.
we know that, whatever "c" and "b" are, we know that
b + c = 70,000.
how much is 13% of b? well, (13/100) * b, or 0.13b
how much is 5% of c? well, (5/100) * a, or 0.05c
we know that, the yield or interest earned from them, must be 5000, thus
0.13b + 0.05c = 5000.
how much did she invest in CDs? well, c = 70000 - b.