A motor car of mass 30kg moving with a velocity of 10m/s-1 by the application of brakes is brought to rest in a distance of 25m.find the average force

F = m a

so what is a ?
a = change in velocity (-10 m/s) / change in time
what is the change in time (how long to stop)
average speed during stop = 10 /2 = 5 m/s
so
time to stop = 25meters / 5meters /sec = 5 seconds
so
a = -10 / 5 = -2 meters/ second^2
F = m a = 30 *-2 = -60 Newtons
By the way, that must be a toy motor car if it only has 30 kg mass.