just do 5 * 8 = 40
so you've got 40. now
see how you have to move the decimal over 3 places to the left to go from 8 to 0.008?
see how you have to move the decimal over 1 place to the left to go from 5 to 0.5?
you have to move the decimal over to the left 4 places (because 3+1=4) in your answer (40)
40. , but move the decimal to the left 4 places makes it 0.0040 or just 0.004