Question 1160717
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1 minute = 60 seconds
60 minutes = 3600 seconds (multiply both sides by 60)
1 hour = 3600 seconds


To convert from 90 km/hr to km/sec, we divide by 3600
This is the same as multiplying by (1/3600)
90 km/hr = (90 km/hr)*(1 hr/3600 sec)
90 km/hr = (90)*(1/3600) km/sec
90 km/hr = 0.025 km/sec


To convert from km/sec to m/sec, we multiply by 1000 since there are 1000 meters in 1 km
0.025 km/sec = (0.025 km/sec)*(1000 m/1 km)
0.025 km/sec = (0.025*1000) m/sec
0.025 km/sec = 25 m/sec


To wrap things up so far,
90 km/hr = 25 m/sec


Vi = initial velocity = 25 m/sec
Vf = final velocity = 0 m/sec
t = change in time = 20 seconds


a = acceleration
a = (Vf-Vi)/t
a = (0-25)/20
a = -25/20
a = -1.25
Negative acceleration indicates the object is slowing down. A counter force of the brakes is being applied opposite of the car's initial direction. 


<font color=red size=4>Average acceleration is -1.25 m/sec^2</font>
Every second, the car slows down by 1.25 m/sec.


Side note: 1.25 m/sec = 4.5 km/hr
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