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To solve the 18th question about the two electric trains, we follow these steps:

**Given:**
- Train A: 
  - Acceleration (\(a_A\)) = 0.2 m/s²
  - Final speed (\(v_A\)) = 45 km/h = 12.5 m/s
- Train B:
  - Starts 1 minute (60 seconds) later
  - Acceleration (\(a_B\)) = 0.4 m/s²
  - Final speed (\(v_B\)) = 72 km/h = 20 m/s

**Find:** When Train B overtakes Train A.

**Steps:**

1. **Time for Train A to reach its maximum speed:**
   \[
   v_A = u_A + a_A \times t_A
   \]
   \[
   12.5 = 0 + 0.2 \times t_A \implies t_A = 62.5 \text{ seconds}
   \]

2. **Distance covered by Train A during acceleration:**
   \[
   s_A = u_A \times t_A + \frac{1}{2} a_A \times t_A^2
   \]
   \[
   s_A = 0 + \frac{1}{2} \times 0.2 \times (62.5)^2 \implies s_A = 390.625 \text{ meters}
   \]

3. **Time for Train B to reach its maximum speed:**
   \[
   v_B = u_B + a_B \times t_B
   \]
   \[
   20 = 0 + 0.4 \times t_B \implies t_B = 50 \text{ seconds}
   \]

4. **Distance covered by Train B during acceleration:**
   \[
   s_B = u_B \times t_B + \frac{1}{2} a_B \times t_B^2
   \]
   \[
   s_B = 0 + \frac{1}{2} \times 0.4 \times (50)^2 \implies s_B = 500 \text{ meters}
   \]

5. **Relative motion after each train reaches its maximum speed:**
   - Train A has constant speed after 62.5 seconds
   - Train B becomes constant speed after 50 seconds

Train B starts 60 seconds later, so:
- Total time for Train A until it is overtaken = t
- Total time for Train B to overtake = t - 60

**Set the equations for the point of overtaking:**

For Train A:
Total distance covered = \(390.625 + 12.5 \times (t - 62.5)\)

For Train B:
After acceleration, distance covered = \(500 + 20 \times (t - 110)\)

Set the two distances equal:
\[
390.625 + 12.5 \times (t - 62.5) = 500 + 20 \times (t - 110)
\]

Solve for \(t\):
\[
390.625 + 12.5t - 781.25 = 500 + 20t - 2200
\]
\[
12.5t - 20t = 500 - 2200 + 390.625 - 781.25
\]
\[
-7.5t = -2090.625
\]
\[
t = 278.75 \text{ seconds}
\]

**Conclusion:**
Train B will overtake Train A 278.75 seconds after Train A starts moving.

Learn how to solve the problem of two electric trains with different accelerations and speeds overtaking each other. This problem involves key kinematics concepts such as acceleration, velocity, and relative motion. Suitable for high school students in Grades 11-12.

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