Q:  How many craters of size 0.1 km should form in 100 million years?

        Well, if we expect 10 craters of this size to form in 1 million years,
        then one hundred times as many 

                100 x 10 = 1000 

        in 100 million years.


  Q:  How many craters of size 1 km should form in 100 million years?

        If we expect 0.001 craters of this size to form in 1 million years,
        then one hundred times as many 

                100 x 0.001 = 0.1 

        in 100 million years.



  Q:  How many craters of size 10 km should form in 100 million years?

        If we expect 0.000 01 craters of this size to form in 1 million years,
        then one hundred times as many,

                100 x 0.000 01 = 0.001 

        in 10 million years.