The last person covered 120.71 meters. 

It is given that the platoon and the last person moved with 
uniform speed. Also, they both moved for the identical 
amount of time. Hence, the ratio of the distance they 
covered - while person moving forward and backword - are 
equal. 

Let's assume that when the last person reached the first 
person, the platoon moved X meters forward. 

Thus, while moving forward the last person moved (50+X) 
meters whereas the platoon moved X meters. 

Similarly, while moving back the last person moved [50-(50-
X)] X meters whereas the platoon moved (50-X) meters. 

Now, as the ratios are equal, 
(50+X)/X = X/(50-X) 
(50+X)*(50-X) = X*X 

Solving, X=35.355 meters 

Thus, total distance covered by the last person 
= (50+X) + X 
= 2*X + 50 
= 2*(35.355) + 50 
= 120.71 meters 


Note that at first glance, one might think that the total 
distance covered by the last person is 100 meters, as he 
ran the total lenght of the platoon (50 meters) twice. 
TRUE, but that's the relative distance covered by the last 
person i.e. assuming that the platoon is stationary.