This. My biggest threat in english was a dude who spent his time playing pokemon in year 7-10 and hitting on chicks all day, and he eventually went on to come 1st.
53.4% is correct.
Logically, there can't be 90% of carbonate in seashells since seashells are mainly comprised on calcium carbonate. Hence it would be a close 50/50 with calcium and carbonate (with some impurities of course)