Geek note: mass and weight are not the same thing. Mass is a measurement of how much matter is in something. Weight is a measure of the pull of gravity on mass. That’s why an object on the moon weighs 1/6 that of that same object on the earth.(And why astronauts on the space station can be weightless and still have body mass.)
Because there are no oceans on the moon to pollute, we will use mass and weight to mean the same thing.
According to multiple scientific sources, the total mass of the water in the oceans is about 1.4 × 1021 kg which is 1.4 x 1018 metric tons
We need to do the math here, so we start with exponents.
150 million metric tons of plastic is 150 x106 metric tons. That is the same as 1.5 x 108 metric tons.
If we divide the mass of plastic (1.5 x 108 metric tons) by the mass of water (1.4 x 1018 metric tons ) we get a pure fraction: 1.07 x 10-10 that means the plastic contamination concentration in the ocean is 11.07 parts per billion. Round that to 11 ppb.
Divide the minimum human contamination of 122 ppb by 11 ppb and you get 11.