Suppose that a scientist estimates that every square mile of the ocean contains an average of 4.6 × 104 pieces of trash. the area of the earth's surface that is covered by oceans is approximately 1.2 × 108 square miles. using the estimate, how many pieces of trash are in the earth's oceans?

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
 
 there are 5.52 * 10 ^ 12 pieces of trash in the earth's oceans