If x is the distance left from 2nd to 3rd, dx/dt = -24 ft/s, x = 20 ft. From the right triangle with side 60 (from 3rd to home) and x, we get the hypotenuse of D = sqrt(602 + x2) (This is the distance to home plate)
Take dD/dt = x dx/dt /sqrt(x2+ 3600)
Plug in all the values for the right-hand-side.
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