The quotes "If you're not part of the solution, you're part of the precipitate," and "Time flies like an arrow; fruit flies like a banana," are both examples of what figure of speech, often used for a humorous effect?

antanaclasis

It is the stylistic trope of repeating a single word, but with a different meaning each time. Antanaclasis is a common type of pun, and like other kinds of pun, it is often found in slogans.

Examples:

A famous example of antanaclasis is seen in William Shakespeare's Henry V when the King sends the French ambassadors back to their master with an answer to the insulting gift of tennis-balls. He says, "for many a thousand widows/ Shall this his mock mock out of their dear husbands; Mock mothers from their sons, mock castles down;" (HENRY V, I, ii, 284-286)

"We must all hang together, or assuredly we shall all hang separately." —Benjamin Franklin

"If you aren't fired with enthusiasm, you will be fired with enthusiasm." —Vince Lombardi

"The long cigarette that's long on flavor." —from an advertisement for Pall Mall cigarettes

https://en.wikipedia.org/wiki/Antanaclasis

I vote for syllepsis.

Example: "You can leave in a taxi. If you can't get a taxi, you can leave in a huff. If that's too soon, you can leave in a minute and a huff. - Groucho Marx, from Duck Soup"

https://en.wikipedia.org/wiki/Syllepsis#Syllepsis

Or maybe the simple 'syntactic ambiguity'.

"A noteworthy example in the field of computer natural language processing is Time flies like an arrow. Although humans unambiguously understand it to mean "Time flies in the same way that an arrow does," it could also mean:

measure the speed of flying insects like you would measure that of an arrow (thus interpreted as an imperative) - i.e. (You should) time flies as you would (time) an arrow.;
measure the speed of flying insects like an arrow would - i.e. Time flies in the same way that an arrow would (time them).;
measure the speed of flying insects that are like arrows - i.e. Time those flies that are like arrows;
all of a type of flying insect, "time-flies," collectively enjoy a single arrow (compare Fruit flies like a banana);
each of a type of flying insect, "time-flies," individually enjoys a different arrow (similar comparison applies);
(As Groucho Marx is said to have observed, "Time flies like an arrow; fruit flies like a banana.")"

https://en.wikipedia.org/wiki/Syntactic_ambiguity

Okay...a pun?

the usually humorous use of a word in such a way as to suggest two or more of its meanings or the meaning of another word similar in sound.

A pun is a figure of speech which consists of a deliberate confusion of similar words or phrases for rhetorical effect, whether humorous or serious. A pun can rely on the assumed equivalency of multiple similar words (homonymy), of different shades of meaning of one word (polysemy), or of a literal meaning with a metaphor. Bad puns are often considered to be cheesy.

http://www.tnellen.com/cybereng/lit_terms/puns.html

Paraprosdokian (Gk. "beyond expectation") used for humor, dramatic effect and producing an anticlimax where the latter part of a sentence or phrase is surprising or unexpected in a way that causes the reader or listener to reframe the first part.

https://en.wikipedia.org/wiki/Paraprosdokian

"I've had a perfectly wonderful evening, but this wasn't it." — Groucho Marx

Voltaire's answer seems the most precise, but Wiki has yet another take on this example:

"A number of jokes depend on the garden path effect, often combined with more usual syntactic ambiguity. For instance, 'Time flies like an arrow. Fruit flies like a banana.' The first sentence starts the hearer firmly down the garden path, priming for a particular parsing of the second sentence which would parallel the first. The joke hinges on the ambiguities of 'fruit' (independent noun or modifier of 'flies'), 'flies' (singular verb or plural noun), and 'like' (preposition or plural verb)."

https://en.wikipedia.org/wiki/Time_flies_like_an_arrow;_fruit_flies_like_a_banana