Explanation:

- Runner covers 40 km in 3 hours; thus, total time = 180 minutes.

- Each quarter is 10 km. The speed ratios are 2:3:4:5 for each quarter.

- Let speeds be 2x, 3x, 4x, 5x km/hr for each 10 km.

- Time for each: Distance/Speed.

1st: 10/(2x) = 5/x hours

2nd: 10/(3x) = 10/(3x) hours

3rd: 10/(4x) = 2.5/x hours

4th: 10/(5x) = 2/x hours

- Sum: (5/x) + (10/3x) + (2.5/x) + (2/x) = 3 hours

- In minutes: (5/x + 10/3x + 2.5/x + 2/x) × 60 = 180

- Combine: (5 + 10/3 + 2.5 + 2) = (5 + 3.33 + 2.5 + 2) = 12.83

- (12.83/x) = 3 ? x ˜ 4.28

- Time for last quarter = 10/(5x) = 2/x hours = (2/4.28)*60 ˜ 28 minutes.

Option 4: 28 minutes