p=you can ride the roller coaster
q=you are under 4 feet tall
r=you are older than 16 years
now see in question question said you can not ride the roller coaster i.e (p')
we can written like that -
you are under 4 feet tall "and" you are not older than 16 years old both statements are same
now simple-
p--> q i.e( q if p ) right .....means before if always came conclusion so in above question before if is statement "p" i.e p will be conclusion i.e p'
now, q
q^r' -->p'