clc;
t=40*1e-2;
//pulse covers 2x distance in arriving back
//so,  30*1e-6=2*x/v
//and, 2nd pulse will cover a distance of 2*40 cm in 80*1e-6 seconds
//therfore,  80*1e-6=(2*40*1e-2)/v
//compare both equation
e1=30;
e2=40*2
x=e1*t*2/(2*e2);
disp(+'m',x,'distanc of the flow from near end =')