Suppose or assume that (some hypothetical were true). Say that you invest $200 into this account. With the interest rate they're offering, you would have $220 by the end of the year. OK, say this melon is the sun, this apple is Earth, and this marble is the moon. Now let's see how they all rotate in relation to one another.
See also: say
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
to assume [something]; to suppose [that something were so]. Say that x is equal to a whole number greater than 10. Say that two trains leave two different cities at the same time.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.