Question 164032
I do not understand how to set this problem up.Please help.Also, how can I check my answer.Thanks in advance. 
Walt made an extra $9000 last year from a part-time job. He invested part of the money at 9% and the rest at 8%. He made a total of $770 in interest. How much was invested at 8%?
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He invested some at 9%, call that N dollars.  The rest at 8%, call the E dollars.

N + E = 9000  
N*.09 + E*.08 = 770
From eqn 1, E = 9000-N
Substitute that into eqn 2
N*.09 + (9000-N)*.08 = 770
.09N + 720 - .08N = 770
.01N = 50
N = 5000
E is 5000-N, or $4000
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To check:
5000*.09 + 4000*.08 ?? 770
450 + 320 = 770, so it's correct.