Bodies in space traveled in circles.
The planets revolved around the Earth.
The figure shows the arrangement of system
The velocity of boat can be resolved in to two
Horizontal component = vcos θ = 2.50 cos 45 = 1.768 m/s
Vertical component = vsin θ = 2.50 sin 45 = 1.768 m/s
Due to horizontal component the boat arrive arrives upstream,
Total horizontal velocity = 1.768 - Vr, where Vr is the velocity of river.
Total time taken to cross the river = width of river/ Vertical component of velocity
t = 285/1.768 = 161.20 seconds
So 118 meter is traveled at a velocity of 1.768-Vr in 161.20 seconds
That is 118 = (1.768-Vr)*161.20
1.768 - Vr =0.732
Vr = 1.036 m/s
So velocity of river flow =1.036 m/s
Let the unknown distance be xmiles
x/39-x/72=11hr
72x-39x/2808=11hr
33x/2808=11
33x= 30888
x=936miles
U can substitue back to check
at speed of 72mph, he would need 936/72=13hrs
at speed of 39mph, he would need 936/39=24hr
the difference is 24-13=11