The North American and European plates of the
Earth’s crust are drifting apart with a relative speed of
about 25 mm/yr. Take the speed as constant and find when
the rift between them started to open, to reach a current
width of 2.9 x 10^3 mi.
width/distance given is 2.9*10^3 miles 1 mile=1.6*10^6mm total distance now is 2.9*1.6*10^9 speed is 25 mm/yr plug the values, therefore the rift takes approximately 185600000 years for a width of 2.9 x 10^3 mi.
