20km every 1 hr is what 20km/hr means.
so 3hrs is 20km*3=60km.
Given :
The focal length of a concave mirror is 18 cm.
To Find :
The radius of curvature of the concave mirror.
Solution :
We know,

Therefore, the radius of curvature of concave mirror is 36 cm.
Hence, this is the required solution.
Potential energy = mass x gravity x height
P.E = 4 x 9.8 x 3
P.E = 117.6 J
Life Expectancy Was Shorter
Trains were faster to ride
The population explosion was not as on date