Atotal of $7000 is invested: part at 8% and the remainder at 14%. how much is invested at each rate if the annual interest is $710?

Let's say the amounts are "a" and "b"

we know the total is 7000, so.. .whatever "a" and "b" are, they add  up to 7000
thus
    a + b = 7000

how much is 8% of "a"? well, 8% of anything is (8/100) * anything
so 8/100 * a or 0.08a

how much is 14% of "b"? well, 14/100 * b or 0.14b

whatever those percentage amounts are, we know, they yielded $710
thus
   0.08a + 0.14b = 710

thus    

solve for "a", to see how much was invested at 8%

what about "b"?  well, b = 7000 - a

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