Acceleration is the rate of change of speed.
If the speed is increased then its called acceleration
If the speed is decresed then its a negative acceleration also called deceleration
the formula for calculating acceleration is
[tex] a = \frac{v-u}{t} [/tex]
where a - acceleration
v - final velocity
u - initial velocity
t - time
v - 15 km/hr should be converted to m/s
distance in 1 hour - 15 km
1 hour = 3600 s
1 km - 1000 m
in 3600 s - 15 000 m
in 1 s - 15 000 m / 3600 s = 4.17 m/s
u - 30 km/hr
in 3600 s - 30 000 m
in 1 s - 30 000 m / 3600 s = 8.33 m/s
substituting the values in the equation
[tex] a = \frac{4.17 m/s - 8.33 m/s}{4 s} = \frac{-4.16}{4} [/tex] = -1.04 m/s²
the answer is negative means its deceleration
the acceleration is -1.04 m/s²