Find all pizzerias that serve every pizza eaten by people over 30

Encountered this Relational Algebra exercise in the Introduction to Databases free online course  offered by Stanford University in Fall, 2011.

Question 9

Find all pizzerias that serve every pizza eaten by people over 30.

(This query is very challenging; extra congratulations if you get it right.)

Person

name	age	gender
Amy	16	female
Ben	21	male
Cal	33	male
Dan	13	male
Eli	45	male
Fay	21	female
Gus	24	male
Hil	30	female
Ian	18	male

Frequents

name	pizzeria
Amy	Pizza Hut
Ben	Chicago Pizza
Ben	Pizza Hut
Cal	New York Pizza
Cal	Straw Hat
Dan	New York Pizza
Dan	Straw Hat
Eli	Chicago Pizza
Eli	Straw Hat
Fay	Dominos
Fay	Little Caesars
Gus	Chicago Pizza
Gus	Pizza Hut
Hil	Dominos
Hil	Pizza Hut
Hil	Straw Hat
Ian	Dominos
Ian	New York Pizza
Ian	Straw Hat

Eats

name	pizza
Amy	mushroom
Amy	pepperoni
Ben	cheese
Ben	pepperoni
Cal	supreme
Dan	cheese
Dan	mushroom
Dan	pepperoni
Dan	sausage
Dan	supreme
Eli	cheese
Eli	supreme
Fay	mushroom
Gus	cheese
Gus	mushroom
Gus	supreme
Hil	cheese
Hil	supreme
Ian	pepperoni
Ian	supreme

Serves

pizzeria	pizza	price
Chicago Pizza	cheese	7.75
Chicago Pizza	supreme	8.5
Dominos	cheese	9.75
Dominos	mushroom	11
Little Caesars	cheese	7
Little Caesars	mushroom	9.25
Little Caesars	pepperoni	9.75
Little Caesars	sausage	9.5
New York Pizza	cheese	7
New York Pizza	pepperoni	8
New York Pizza	supreme	8.5
Pizza Hut	cheese	9
Pizza Hut	pepperoni	12
Pizza Hut	sausage	12
Pizza Hut	supreme	12
Straw Hat	cheese	9.25
Straw Hat	pepperoni	8
Straw Hat	sausage	9.75

These slides explain how to formulate relational algebra queries like the one presented above.