91Ó°ÊÓ

Given and known data:

The echo time is t = 2.56 sec.

The speed of radio wave travel = \(c = 3 \times {10^8}\;{\rm{m}}/{\rm{s}}\).

The echo time is the time lag between the transmitting and receiving ends. This duration increases with the distance between the sender and the receiver.

The distance between the Earth and the Moon is given by

d = c.t

Here, d is the distance between the Earth and the Moon, c is the speed of light, and t is the echo time.

Substitute all the values in the above equation.

\(\begin{array}{l}d = \left( {3 \times {{10}^8}\;{\rm{m}}/{\rm{s}}} \right)\left( {2.56\;{\rm{s}}} \right)\\d = 7.68 \times {10^8}\;{\rm{m}}\end{array}\)

Therefore, the distance from Earth to the Moon for echo time is \(7.68 \times {10^8}\;{\rm{m}}\).