Atypical deposit of cholesterol, c27h46o, in an artery might have a mass of 3.9 mg. how many molecules of cholesterol are in this mass?

1)you need first to find the molar mass.
To find it,  you have to consult the periodic table and see the atomic mass of each element. If the element is repeated, for example 3 times, you multiple the atomic mass per 3.

molar mass of C27H46O: 27 x12 +46 x 1 + 16= 386, 65 g/mol

2) to discover the number of molecules, is necessary to find the number of moles. You use a formula that says that moles is calculated by dividing mass by molar mass.
moles: x
molar mass: 386,65 g/mol
mass= 3,9 mg but needs to be converted into  grams, so: 0,0039 grams
 x= 0,0039/386,65
x=1,0087 x 10⁻⁵ moles

3) finally, we have all that's necessary to calculate the number of molecules.
We just need to use a formula that relates moles with molecules, and it's the following: number of molecules= moles x the avogadro number
(the avogadro number is a constant)

1,0087 x 10⁻⁵ moles   x 6.022 x 10²³ =6.1x10¹⁸ molecules of cholesterol.