a tissue sample weighs 150 milligrams and contains 75x10 E 8 cells what is the mass of each cell in grams
150 milligrams is 150*10-3 grams, so you have:
$$\frac{150\times10^{-3}}{75\times10^8}\rightarrow 2\times10^{-11}$$ grams per cell
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+97 
