Amolecular model scale uses 2.5 cm: 0.1 mm. if the model is 7 cm long, how long is the molecule?

We can answer this problem using a rule of three simple, direct proportion:
2.5 cm > 0.00001 mm
7 cm >  x
x = (7)(0.00001)/2.5
x = 0.000028
Therefore the molecule is 0.000028 mm long, that number can be also written using scientific notation to avoid such quantity of zeroes, 2.8x10^-5 mm, that is 2.8 times 10 raised to the -5 power