Velocity = distance/time
Tangential velocity is the distance covered around the arc of the circular path per unit time.
The period is the time taken to cover one revolution and it is equal to the circumference of a circle. This means that the distance covered is 2pi x radius
Thus,
Tangential velocity = 2 x pi x radius/period
From the information given,
period = 2 x 10^7 s
radius = 4 x 10^10 m
pi = 3.14
Thus,
Tangential velocity = 2 x 3.14 x 4 x 10^10/2 x 10^7
Tangential velocity = 12560 m/s