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 backward - 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,

$\frac{[50+X]}{X}=\frac{X}{[50-X]}$
$[50+X]\times [50-X]=X\times X$

Solving, X=35.355 meters

Thus, total distance covered by the last person 
$=[50+X]+X$
$=2\times X+50$
$=2\times \left[35.355\right]+50$
=120.71 metres.
=120 m (Approximately)