Boron makes up approximately 15.944% of the mass and the rest of the 84.056% is Fluorine. There is 5 Atoms because Boron atomic mass is 10.811 in 1 molecule of BF3 and you wanted 5 Molecules.
it's because there is no number written next to the symbol 'B' so it means there's one boron so if we multiply it by 5 since there are 5 molecules it will give us the answer '5'.
