**Calculating the Retardation**

To calculate the retardation of the truck, we need to first calculate its initial velocity and final velocity.

Given:
Initial velocity (u) = 60 m/s
Time taken to bring the truck to rest (t) = 5 s
Final velocity (v) = 0 m/s

We can use the formula for acceleration (a) to calculate the retardation:

a = (v - u) / t

Substituting the given values:

a = (0 - 60) / 5

a = -12 m/s^2

Therefore, the retardation of the truck is -12 m/s^2.

**Calculating the Distance Travelled**

To calculate the distance travelled by the truck in 15 seconds, we need to consider two intervals: the first 10 seconds where the truck travels at a uniform velocity, and the next 5 seconds where it comes to rest.

1. Distance travelled in the first 10 seconds:

Given:
Initial velocity (u) = 60 m/s
Time taken (t) = 10 s

Using the formula for distance (s) covered at a uniform velocity:

s = u * t

Substituting the given values:

s = 60 * 10

s = 600 m

Therefore, the distance travelled in the first 10 seconds is 600 meters.

2. Distance travelled in the next 5 seconds:

Given:
Initial velocity (u) = 0 m/s (as the truck is at rest)
Time taken (t) = 5 s

Using the formula for distance (s) covered under constant retardation:

s = u * t + (1/2) * a * t^2

Substituting the given values:

s = 0 * 5 + (1/2) * (-12) * (5^2)

s = -30 * 6

s = -180 m

Since distance cannot be negative, we take the magnitude:

s = 180 m

Therefore, the distance travelled in the next 5 seconds is 180 meters.

**Total Distance Travelled in 15 Seconds**

To calculate the total distance travelled in 15 seconds, we add the distances covered in both intervals:

Total distance = Distance in the first 10 seconds + Distance in the next 5 seconds

Total distance = 600 m + 180 m

Total distance = 780 meters

Therefore, the truck travels a total distance of 780 meters in 15 seconds.

retardation=4m/s^2