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Hi,
Let x and y represent amount saved at 10% and 12% respectively
x + y = $8,000 0r 
.10x + .12y = $900
.10(8,000 - y) + .12y = 900 |solving the system of EQs by substitution
.02y = 900 - 800
.02y = 100
y = $5000 and x = $3000
CHECKING our answer***
= $900
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