Actually Welcome to the concept of General organic chemistry.
1.) The formula for alkanes is Cn H2n+2
===> here n = 500, hence we get as,
C500, 2n+2 = 2(500)+2 ==> 1000+2 ==> 1002
hence the formula is ==> C500 H1002
2.) The formula for alkenes is,
CnH2n
hence, here n = 15
so we get as,
===> C15 H30