Hi John,
Let x = the amount invested in the CD
then 13000-x = amount invested in the savings bond
and we know the interest for each investment type and the total interest so we can form the following equation
$780 = 4% (amount in CD) + 7% (amount in bond)
780 = .04 x + .07 (13000-x)
780 = .04x + 910 - .07x
780 = -.03x = 910
-130 = -.03x (subtract 910 from both sides
4333.33 = x (divide both sides by .03 and this is the amount in the CD
13000-x = 8666.67 (the amount in the bond
Hope this helps,
Tom