2x2 System word problem?

An underwater(but near the surface) explosion is detected by sonar on a ship 30s before it is heard on deck. If sound travels at 5,000 ft/s in water and 1,000 ft/s in air, how far is the ship from the explosion?

All I need is the equations and I'll be aright to go from their.

Let be equal to the time it takes to be heard by the sonar.

We then have that the time for it to be heard on deck is .



Therefore:



And since the distances are equal:



Now solve for to get:





Now just plug that into the equation to get:



feet