Let d1 and d2 be the distances of cliffs from the boy .
for first echo , which is heard after 2s ,
total distance travelled by sound =2d1 ,
given v=330m/s ,
we have , distance=speed×time ,
2d1=330×2 ,
or d1=660/2=330m ,
for second echo , which is heard after 2.5s ,
total distance travelled by sound =2d2 ,
given v=330m/s ,
we have , distance=speed×time ,
2d2=330×2.5 ,
or d2=825/2=412.5m ,
therefore , distance between cliffs , d=d1+d2=330+412.5=742.5m