A rock star who works part time as chemist, invests a total of $12,000 in 2 accounts. One account earns 4% interest per year; the other account earns 7% per year. If the total interest she earns in one year equals $780, how much does she have invested in each account?
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x is total in 4% account. So $12,000 - x is in the other account.
4% * x + 7% * ($12,000 - x) = $780
0.04x + 0.07*(12,000 - x) = 780
0.04x + 840 - 0.07x = 780
0.04x - 0.07x = 780 - 840
-0.03x = -60
x = -60/(-0.03) = $2,000 in the 4% account, so there is $10,000 in the 7% account
4% * x + 7% * ($12,000 - x) = $780
0.04x + 0.07*(12,000 - x) = 780
0.04x + 840 - 0.07x = 780
0.04x - 0.07x = 780 - 840
-0.03x = -60
x = -60/(-0.03) = $2,000 in the 4% account, so there is $10,000 in the 7% account