The amount of you swam is +15 feet and your friend swam is -6 feet.
Given that you move forward 15 feet and your friend is not a strong swimmer, so he moves back 6 feet.
The arithmetic operators means performing an addition, subtraction, multiplication, division, exponentiation, and modulus operations.
It is given that you move the forward that means you move in positive direction.
So, you move forward means you move +15 feet.
It is also given that your friend is not a strong swimmer, so he moves back that means he move in negative direction.
So, your friend moves backward means he move -6 feet.
Hence, you move forward 15 feet is +15 feet and your friend is not a strong swimmer, so he moves back 6 feet is -6 feet.
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