book Maths Puzzle Numerical Ability

(((x-10)/(.5(x-10)))100)/10%=.4
I know that the answer is 9.6 but I can not figure out how.

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Tom invested 13,000.00 for one year, part at 9 percent annual interest and the rest at 7 percent annual interest. How much must he invest at 9 percent to get $1000.00 in interest in a year? This new math sure gets me. I am a senior and trying to answer this question.
[2x(9+7)-4]/7=
Thank you.