Answer: The amount invested at 10% was $3200.
Step-by-step explanation:
Since we have given that
Principal amount = $7200
Rate of interest are 10% and 9%.
Total interest = $680
Let the amount invested on 10% be 'x'.
Let the amount invested on 9% be '7200-x'.
According to question, we get that
[tex]\dfrac{x\times 10}{100}+\dfrac{(7200-x)\times 9}{100}=680\\\\10x+64800-9x=680\times 100\\\\x+64800=68000\\\\x=68000-64800\\\\x=\$3200[/tex]
Hence, the amount invested at 10% was $3200.