Finding the speed of an approaching object using the doppler effect and sonar

[jellyman]

Homework Statement

I attatched 2 images, the problem, and my work (including the formula needed)This "double shift" is what's getting me. Is it going 2% towards the object, and 2% from it?

Or I'm I suppose to use 2 functions:F'=(f+xf) and F'=(f+yf) where x + y = 0.04?

Homework Equations

See pic.

The Attempt at a Solution

See pic.

 

[voko]

Do as the hint suggests - in two steps.