Answer:
4.2 mm/mo
Step-by-step explanation:
[tex]\dfrac{2400\,mi}{76\cdot 10^6\,yr}\cdot\dfrac{1.609344\cdot 10^6\,mm}{1\,mi}\cdot\dfrac{1\,yr}{12\,mo}=\dfrac{2400\cdot 1.609344\cdot 10^6\,mm}{76\cdot 10^6\cdot 12\,mo}\\\\\approx\dfrac{4.2\,mm}{mo}[/tex]