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"84%" is read as "84 percent",
which literally means "84 per century" or "84 per hundred."

So the given statement is actually saying this :
About 84 out of every 100 smart phone users notice ads on their phone.

84 out of 100 is written as \dfrac{84}{100} and we say "eighty four percent."
It means that 84 per cent of people or 84 per century which means 84:100 or 84 for people get ads on their phones out of 100 people