1. The problem asks to write $0.40 as a fraction of a dollar. 2. Recall that 1 dollar equals 1 whole, so $0.40 means 40 cents out of 100 cents. 3. We can write $0.40 as the fraction $\frac{40}{100}$. 4. Simplify the fraction by dividing numerator and denominator by their greatest common divisor, which is 20: $$\frac{\cancel{40}^{2}}{\cancel{100}^{5}} = \frac{2}{5}$$ 5. Therefore, $0.40$ as a fraction of a dollar is $\frac{2}{5}$.