Answer
$x=9$
Work Step by Step
Let the unknown number be $x$. Since, the sum of $x$ and two-thirds of $x$ is 15, we obtain:
Step 1: $x+\frac{2}{3}x=15$
Step 2: $ \frac{3x+2x}{3} =15$
Step 3: $ \frac{5x}{3} =15$
Step 4: $5x=3(15)$
Step 5: $5x=45$
Step 6: $x=9$
