Answer:
400
Explanation:
Because if you pull one black marble out of four and there are 100 black marbles then there is 300 others, so in total it would be 400 marbles.
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Answer: The length of the cell in millimetres is 0.0015.
Explanation:
Given conversion :

Thus 
Given : Length of the cell = 1.5 micrometers 
To find: Length of the cell in milllimeters (mm)
Length of the cell in milllimeters (mm) = 
The length of the cell in millimetres is 0.0015.