Write 87% as a decimal.

The definition of a percent is in its name: per cent, where cent stands for a hundredth of a whole. (Much like a cent or penny is 1/100 of an American dollar, or a centurion was one of a hundred Roman soldiers, etc...)
So 87% is 87 parts of a whole comprised of a hundred parts.
87%=87/100 =0.87
 
http://mathworld.wolfram.com/Percent.html