v = average speed of movement of the Southwest Indian Ridge = 20 mm/year
d = distance moved by the Southwest Indian Ridge = 100 mm
t = number of years required to move distance "d"
distance traveled is given as
d = v t
inserting the above values in the formula
100 mm = (20 mm/year) t
dividing both side by 20 mm/year
t = 100 mm/(20 mm/year)
t = 5 years