The distance travelled by a falling body in the last second of its motion, to that in the last but one second is 7 : 5, the velocity (in m/s) with which body strikes the ground is $\_\_\_\_\_ \times {10}^{-1}$

Distance travelled by a freely falling body in nth second is 

Sn = ½ g [2n-1]

Given ratio of distances travelled in nth second and (n-1)th second as 7/5

⇒[g(2n-1)]/[g(2n−3)] = 7/5

⇒n = 4

v = gt

   =39.2m/s