If man were standing unsymmetrically between parallel cliffs, claps his hands and starts hearing a series of echoes at intervals of 1 s. If speed of sound in air is 340ms⁻¹, the distance between two cliffs would be 510 m, 170 m, 340 m, or 680 m?

Let the distance between the cliffs be l and x be the distance between man and nearest cliff.
The man hears the first echo after 1s. During this time, the sound travels upto the nearest cliff and reflects back to the man.
Thus, 2x = 340 * 1 => x = 170m
The man hears the second echo after 2s, or 1s after the first echo. During this time (2s), the sound travels from man upto the farther cliff and returns to the man.
Thus, 2(l - x) = 340 * 2 => l - x = 340 => l = 340 + x = 340 + 170 = 510m