Assume that the original speed of the troops is x km/h and the total travel distance is y km.

It takes x/y hours to reach the original speed.

Y/x-y/(x+1/9x) = 1/3 ...1(if the speed is increased to1/9, it will arrive1/3 hours earlier).

Y/x-72/x-(y-72)/(x+1/3x) = 1/2 ... 2 (The original speed was 72km, then the speed was increased to1/3, and it arrived1/2 hours earlier.

Y =180km. (x=54 km/h) is obtained from the formula 1.2.