Miguel M. answered 12/03/13
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Assuming constant water density, pressure increases by 1 atm ( about 100.9 kPa ) with each 10 m of depth below the surface. So at 10 m, the pressure is 2 atm (1 atm of air + 1 atm of water) or 201.8 kPa.
At 18 atm, the pressure is 282.3 kPa. Now for Boyle's law, you now have P2 (282.3 kPa) and P1 (100.9 kPa). V1 is unknown, but that's OK. You just want the fraction that the volume decreased, so V2 = V1(P1/P2),
the fraction filled with air is P1/P2.
Mckenzie L.
12/03/13