1. Stating the problem: Simplify the expression $\frac{9}{10} - \frac{3}{5}$.\n\n2. Find a common denominator: The denominators are 10 and 5. The least common denominator (LCD) is 10.\n\n3. Convert $\frac{3}{5}$ to have denominator 10:\n$$\frac{3}{5} = \frac{3 \times 2}{5 \times 2} = \frac{6}{10}$$\n\n4. Rewrite the expression with common denominators:\n$$\frac{9}{10} - \frac{6}{10}$$\n\n5. Subtract the numerators while keeping the denominator:\n$$\frac{9 - 6}{10} = \frac{3}{10}$$\n\n6. Final answer: $\frac{3}{10}$.