Question 1031863
Forward time - return time = 40 minutes = 2/3 hours

Let forward rate be x
return rate = x+4

16/x - 16/(x+4) = 2/3

LCD

3x(x+4)

48(x+4) -48x = 2*3x(x+4)

48x + 192 -48x = 6x^2+24x

6x^22 +24x -192=0

/6
x^2+4x-32=0

x^2+8x-4x-32=0

x(x+8)-4(x+8) =0

(x+8)(x-4) =0

x=4 the positive value

4 mph & 8 mph