Using proportions, it is found that Bob will be 90 feet from the surface of the water after 2 minutes.
A proportion is a fraction of a total amount, and the measures are related using a rule of three.
In this problem, every half-minute, he ascends 15 feet. Hence in 2 minutes = 4 half-minutes, he will have ascended 15 x 4 = 60 feet.
He is 150 feet below the surface of the water, hence after 2 minutes he will be 150 - 60 = 90 feet from the surface of the water.
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