Help - Relational Algebra expressions

I've been banging my head against the wall, the table and the kitchen floor the whole day, and I need help.

Is there any alternative to using the select operator in order to filter out specific tuples that satisfy certain conditions?

Generally, if the conditions are given explicitly, select wouldn't be a problem, but I can't use it for cases where the conditions are implied/abstract (all names starting with "M" instead of Name="Mary", for example Actually that's a bad example since I imagine it's easily solvable with regex; so, say, all names which are longer than "John").

 Original problem

Given these three relations/tables:
Person

name	age	gender
Mary	16	female
Susan	19	female
Sal	32	male

Eats

name	pizza
Mary	cheese
Susan	cheese
Susan	mushroom
Sal	cheese
Sal	supreme
Sal	mushroom

Serves

pizzeria	pizza	price
Pizza Hut	cheese	9.75
Pizza Hut	mushroom	10.5
Straw Hat	supreme	12
Dominos	mushroom	11

Find all pizzerias that serve [b]every pizza eaten by people over 30[/b].

Problem is I don't know how to establish the "every" condition, and I can't perform a selection with the pizzas as conditions, since I don't know how many pizzas there are.