1. **Problem statement:** A child riding a bicycle at 15 m/s decelerates at the rate of 3.0 m/s² for 4.0 seconds. We need to find the child's speed at the end of 4.0 seconds. 2. **Formula used:** The final velocity $v$ after time $t$ with initial velocity $v_0$ and acceleration $a$ is given by: $$v = v_0 + at$$ 3. **Important rules:** - Deceleration means acceleration is negative. - Units must be consistent. 4. **Substitute the values:** $$v = 15 + (-3.0)(4.0)$$ 5. **Calculate:** $$v = 15 - 12$$ 6. **Final answer:** $$v = 3\ \text{m/s}$$ The child's speed at the end of 4.0 seconds is 3 m/s.