**Solution:**

Since, the three persons start walking together.

Therefore, The minimum distance covered by each of them in complete steps = LCM of the measures of their steps

= LCM (40, 42, 45)

40 = 2 x 2 x 2 x 5 = 2^{3} x 5

42 = 2 x 3 x 7

45 = 3 x 3 x 5 = 3^{2} x 5

=> LCM (40, 42, 45) = 2^{3} x 3^{2} x 5 x 7

= 2520

=> Each person should walk a minimum distance of 2520 cm in complete steps.