Vague generalization, but 1.8s are often cheap plastic lenses. Whereas 2.8s are more often pro quality zooms.

The cheap plastic lenses can have awful aberration and muddy color and sharpness wide open, especially in low light, therefore to get the best quality out of your lens, you’ll probably be closer to 2.8 anyways.

1.4s, however, are much better lenses than 1.8s, and primes are generally better than zooms, but at the pro level it’s really hard to distinguish 2.8 primes vs 2.8 zoom.

With primes, I shoot around 1.8-2.2 often, and 1.4 in low light. 50 at 2.0 is probably my favorite look, but all primes look beautiful at 2.0.

Lastly, 1.4 to 2.8 is four times the amount of light transmission. That’s a big jump when you’re in the lowest light environments.

Going by BODMAS/PEMDAS, the brackets need to be solved first. So, we get 2+5*3. As multiplication takes priority, we get 2+15 which gives 17.

If you're adding 2+5 and multiplying it with (8-5) to get 21, that is incorrect. You can't add 2+5 without multiplying the 5 into the brackets. On expanding the brackets too, you get 2 + 40 - 25 which again gives 17

(8÷2) (2+2)

=16

8 ÷ (2(2+2))

= 1

These are two completely different questions with two completely different answers. The symbols that you have typed are intentionally ambiguous.

It's like asking "who is the president?"

Well, it depends on what you mean by "the president". We can argue all day about whether we ought to implicitly assume you're talking about the current president of the US or the current president of France, but ultimately it's not the readers job to decipher what was probably meant, it's the writers job to make themselves clear.

====================

Ultimately, this is how you should approach such problems (depending on the scenario)

scenario a: someone has handed you this problem to solve.

Ask that person to clarify what calculation they are asking you to perform.

scenario b: you have found this problem in an exam

BEFORE THE EXAM, ensure you know what the exam board expects you to do in these situations and follow that advice to the letter.

scenario c: some social media ppst gave you this problem

Ignore them. They are intentionally writing in an ambiguous way to make people argue over literally nothing.