A word problem i need to be shown how to figure it out.?

Question:Bob invested $20,000, part at 14% and part at 13%. If the total interest at the end of the year is $2,720, how much did he invest at 14%?

Answers:
Use this chart that I've made fora nearly identical problem.
http://www.flickr.com/photos/18955982@n0...
a+b=20,000 -> b=20,000 - a (substitute this for b)
.14a+.13b=2720

.14a+.13(20,000 - a) = 2720
solve for a
.14a+2600 - .13a=2720
.01a+2600=2720
.01a=120
solve
you do 0.14x+0.13(20000-x)=2720
solve.
0.14x+2600-0.13x=2720
simplify and add like terms.
0.01x+2600=2720
minus 2600 from both sides (simplify)
0.01x=120
x=12000

Bob invested $12,000 at 14% and $8,000 at 13%

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