1. Each right-hand flag (i.e. in the sailors’ right hands) represents one of the four basic mathematical operations: +, -, x, /.
2. Each left-hand flag has stars on it.
3. By starting with the letter shown in semaphore, and applying the operation and the number represented by the stars, you get a new number. So for example, the first sailor is in an M shape, which is the 13th letter. His right-hand flag shows a plus. His left-hand flag has 8 stars. So the operation to do is 13 + 8, which is 21. Which coverts to a U.
4. In order, the answer is USA RUSSIA CHINA. Using the same conventions as the puzzle, that can be translated into 50 - 5. (Fifty stars / minus sign / five stars.) The answer is 45, or FORTY FIVE.