Let the individual speeds of bike 1, 2 and 3 be ‘x1’, ‘x2’ and ‘x3’ km/hr respectively

When two bodies are moving towards each other, i.e. moving in opposite direction, their relative speed is the sum of their individual speeds

While, when two bodies are moving in the same direction, their relative speed is the difference of their individual speeds

Now, let,

Relative speed of bike 1 and 2 = ± x1 ± x2 = 35      ----(1)

Relative speed of bike 2 and 3 = ± x2 ± x3 = 75      ----(2)

Subtracting (1) from (2), we get,

⇒ (± x2 ± x3) - (± x1 ± x2) = 75 - 35

⇒ ± x2 ± x3 ? x1 ? x2 = 40

⇒ ± x3 ? x1 = 40

⇒ ± (x3 - x1) = 40 = Relative speed of bike 1 and 3

? Relative speed of bike 1 and 3 is equal to the difference of their individual speeds