Avi's blood test shows that there are 5 × 106 red blood cells in 1 mm3 of his blood. Using a calculator, you would find that there are E red blood cells in 1 liter of his blood. (Hint: One liter is equal to 1 × 106 mm3.) NextReset

We know that there are 5 x 10^6 red blood cells in 1 mm^3 of Avi`s blood. And 1 liter = 1 x 10^6 mm^3.
There are E red blood cells in 1 liter of his blood.
E = 5 x 10^6 x 1 x 10^6 

E = 5 x 10^12 blood cells ( or : 5,000,000,000,000 ).