If 85% sits underwater, then that means 15% (100-85=15) lies above water. To find 15% of 150000, first find the highest common factor between 15 and 100. It is 5. Now, divide 100 to get to 5. You have to divide by 20. Also divide the total mass by 20 to get 7500. Now we know that 5% of the total mass is 7500. Multiply that figure by 3 to get 15% of the total mass. 7500x3=22500.
Answer: 22500
I apologise if my answer wasn't entirely clear.