constitutional isomers

Question

How many possible constitutional isomers of cycloalkane having the molecular formula C5H10?
How many possible constitutional isomers having the molecular formula C6H14?
How many possible constitutional isomers of alkene having the molecular formula C5H10?

請問Michael第一題的答案：dimethylcyclopropane (a pair of geometrical isomers) 何解？答案不是五個嗎？

Answer 1

How many possible constitutional isomers of cycloalkane having the molecular formula C5H10?

4 constitutional isomers :
cyclopentane
methylcyclobutane
ethylcyclopropane
dimethylcyclopropane (a pair of geometrical isomers)

=====
How many possible constitutional isomers having the molecular formula C6H14?

5 constitutional isomers :
hexane
2-methylpentane
3-methylpentane
2,2-dimethylbutane
2,3-dimethylbutane

=====
How many possible constitutional isomers of alkene having the molecular formula C5H10?

6 constitutional isomers :
pent-1-ene
pent-2-ene
2-methylbut-1-ene
3-methylbut-1-ene
2-methylbut-2-ene
2,2-dimethylpropene

Answer 2

I DON'T KNOW~~