For this case we can make the following rule of three:
1 square mile -------------------> 4.6 * 10 ^ 4 pieces of trash
1.2 * 10 ^ 8 square miles -------> x
From here, we clear the value of x:
We have then:
x = (1.2 * 10 ^ 8) * (4.6 * 10 ^ 4)
x = 5.52 * 10 ^ 12 pieces of trash
Answer:
there are 5.52 * 10 ^ 12 pieces of trash in the earth's oceans