Answer
$10000$ dollars at $4.5\%$ and $15000$ dollars at $3\%$.
Work Step by Step
Let $x$ be the amount invested at $4.5\%$ interest. Then the amount invested at $3\%$ interest is $25000-x$. Then our equation is: $0.045x+(25000-x)\cdot0.03=900\\0.045x+750-0.03x=900\\0.015x=150\\x=10000.$
Thus I invested $10000$ dollars at $4.5\%$ and $15000$ dollars at $3\%$.
