Donald's Lunch 1

Donald McRonald often has hamburgers for lunch. The total benefit he gets depends on the number of burgers he eats per week. Here is the relationship between burgers per week and total benefit per week:
burgers benefit in dollars
115
222
326
428
529
629

This means, for example

Donald would sell a hamburger for from 15 to 29 dollars
Donald would give up goods and services worth 26 dollars to get 3 burgers
the fourth burger is worth more than the third burger
returns to scale are increasing
Donald is irrational -- being a clown