Physics Question

[mikeczyz]

This is really more math but I can't figure it out:

A person, with his ear to the ground, sees a huge stone strike the concrete pavement. A moment later two sounds are heard from the impact: one travels in the air and the other in the concrete, and they are 0.75 seconds apart. How far away did the impact occur? Assume speed of sound in concrete is 3000 m/s and speed of sound in air is 343 m/s.

Clearly, the sound traveling through the concrete will be heard first. I tried to relate it to the distance=velocity*time equation but that did not yield satisfactory results. I built a spreadsheet and I think I have the right answer but would like to know how to do it using only pencil and paper.

Thanks.