Correct Answer - Option 1 : 25 cm

**Concept:**

Sleeper density is denoted as L + m.

Where L=Length of one rail; m= any integer

Sleeper density signifies the number of sleepers per one rail length.

Optimum depth of ballast cushion = (S-W)/2

Where S = Spacing of sleepers

W=Bottom width of sleeper

**Calculation:**

Sleeper density = (M + 5) and bottom width = 22.22 cm

Length of one rail = 12.8 m

No of sleepers = 12.8 + 5 = 17.8

Spacing of sleepers = (12.8x100)/17.8 = 71.91 cm

Optimum depth of ballast cushion=(S-W)/2 = (71.91 - 22.22)/2 = 24.84 cm = 25 cm