A scuba tank is filled with air to a pressure of 231 atm when the air temperature is 30 degrees C

A diver then jumps into the ocean and, after a short time treading water on the ocean surface, checks the tank's pressure and finds that it is only 221 atm. Assuming the diver has inhaled a negligible amount of air from the tank, what is the temperature of the ocean water?

Thanks!!

This is a job for Amontons' Law.
P1/T1 = P2/T2

Let T2 be the ocean temperature. Isolating T2 in the formula:

P1T2 = P2T1
T2 = P2T1/P1
T2 = (221 atm)(303 K) / 231 atm
T2 = 289.88 K or 16.88°C (Answer)

Have a good day.