Answer:

A stationary wave which is at its mean position when the particles at t = T/4 and 3T/4 are at rest.

The displacement of medium particles at distances of 10, 20, 30, 40, and 50 cm is always zero, which is a feature of nodes in a stationary wave. Therefore, we can write:

λ/2 = 30 – 20 = 10

λ = 20 cm