How many C atoms are there in 5.25 grams of a compound containing 92.26% C and 7.74% H?

Take 5.25 grams and give it proportionally to the carbon

0.9226 * 5.25 grams = 4.84 grams Carbon

Find the number of moles

(4.84 grams carbon) / (12.01 grams/mol) = 0.403 moles Carbon

The number of atoms in a mole of a substance is called Avogadro's number.

There are 6.022 * 10^23 atoms in one mole of anything

(0.403 moles Carbon) * (6.022 * 10^23 atoms/mole) = 2.43 * 10^23 atoms of Carbon