The word most opposite in meaning to sentinel is:

A 130-pound woman is wearing shoes with high heels that measure 1-inch square. If the woman is standing on one heel, what psi does the heel exert as it rests on the ground? (Disregard atmospheric pressure from your calculations.)

The water was calm that day with detritus slowly moving in the small eddies.

Defective most nearly means:

A student buys a science textbook for $18.00, a math textbook for $14.50, and a dictionary for $9.95.
What's the total cost of the books?