To calculate percentages, take the amount that fits your criteria, divide by the whole thing, then multiply by 100%. In this case, we would take 5,003 people over the age of 18, divide by the whole crowd of 5,721 people, then multiply by 100%:
5003/5721 * 100% = 87.45%, then round to the nearest whole-number: 87%.
I hope this helped!