The term "butyl cyclopentane" is a bit vague but I am assuming that you are talking about a chemical with a cyclopentane core and a linear, unbranched butane group attached to it. Normally we look for the longest unbroken carbon chain but in the case of cycloalkanes we should treat the cyclopentane as a single unit. So since cyclopentane has 5 carbons and butane has 4 carbons, the name will be based on cyclopentane. The butyl group will be treated as a substituent. Since there is only one substituent off of it, that carbon will be carbon #1. The linear form of butane is called n-butane, so the IUPAC name for the chemical is 1-n-butylcyclopentane.
I cannot draw the structure here, but the cyclopenane would be drawn like a pentagon with a carbon in each corner. At one of the carbons, draw a 4 carbon chain for the n-butyl group.