Hello,

Optical Path of monochromatic light = μt

Given μ1 = 1.510, t1 = 2 cm

μ2 = 1.760, t2 = X cm

Since Optical Path is same, μ1 x t1 = μ2 x t2

1.510 x 2 = 1.760 x X

3.02 = 1.760 x X

3.02 / 1.760 = X

1.7159 = X

1.716 cm = X (After Roundoff)

Therefore, The Optical path of a monochromatic light is same if it goes through 2 cm of glass or 1.7159 cm of ruby

Hope this was helpful.