can someone give me a formula for this please?
A college student earned $8400 during summer vacation working as a waiter in a popular restaurant. Part was invested at 8% simple interest and the remainder at 9% simple interest. At the end of one year, the student had earned $707 interest. How much was invested at 9%

Question

owlfred · Answer

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anonymous · Answer

707=0.08(8400 +0.09(8400)

anonymous · Answer

not possible

amistre64 · Answer

the formula(s) would be:

 a + b = 8400;   a = 8400-b

 707 = a(.08) + b(.09) ; substitue the 'value' of a into this equation and solve for 'b'

anonymous · Answer

oops yes it is sorry. call the amount invested at 9% x. then 
\.09x+.08(8400-x)=707\]

anonymous · Answer

$$.09x+.08(8400-x)=707$$
$$.01x+672=707$$
$$.01x=35$$
$$x=\frac{35}{.01}=3500$$