Triggered by the question how to DM me… Since there isn't some (easy) to DM each other at this moment, I have searched the "internets". ;-)
ZKP (zero-knowledge proof) might be interesting, but we don't have a protocol.

One way to let others know that you have found a solution, without actually revealing what you have found, is to use cryptography and then (publicly) post your encrypted solution.
When the time comes to reveal the solution (the answer that will be posted by the quizmaster) and you want to unveil your own solution, you post the key to your encrypted solution, so that all others can decode/decrypt and check your solution.

Another way would be to use steganography: the prover picks a picture (or a video), already available on the internet, but doesn't disclose its location, then posts the places of the characters in the picture that are part of the solution in the correct order. Later on the prover discloses the location of the picture, thereby revealing their answer and proving that the answer is right.
(If you pick a video, it should be downloadable (by open source programs), e.g. by the program yt-dl.)

Just a wild thought.
Adri

Explanations:

（a） 10 × 100 - (3 × 25 + 6 × 8)

1000 seems a good starting point:
10 × 100 = 1000; we need to subtract 123 to reach 877 using 25, 3, 6 and 8.
First, multiply 25 and 3 (result: 75); subtracting 75 from 123 gives 48.
Second, multiply 6 and 8 (result: 48). Adding 75 and 48 gives 123.
Subtract products 75 and 48 from product 1000. Result: 877.

（b）10^3 - (100 + 25 + 6 - 8)

1000 seems a good starting point:
Raise 10 to the third power, using figures 10 and 3, notation: 10^3, yielding 1000.
We need to subtract 123 from 1000 to yield 877, using only the remaining four figures 100, 25, 8 and 6.
Add 100 and 25, which is close to 123, so we need an additional value of 2 to subtract: 8 minus 6. Result: 877.

Adri

Nice puzzle. I tried a couple of things, but didn't really have time to dive in like I would have liked. Too many other things going on. Even if I don't solve them, I like the challenge of making my brain think.
Thanks

This weekend's puzzle. Wishing you good luck.

What is the value of the question marks?

┏━━━┯━━━┯━━━┓┏━━━┯━━━┯━━━┓┏━━━┯━━━┯━━━┓┏━━━┯━━━┯━━━┓┏━━━┯━━━┯━━━┓┏━━━┯━━━┯━━━┓
┃   │   │ o ┃┃   │   │   ┃┃   │   │   ┃┃   │   │   ┃┃   │   │   ┃┃ o │   │   ┃
┠───┼───┼───┨┠───┼───┼───┨┠───┼───┼───┨┠───┼───┼───┨┠───┼───┼───┨┠───┼───┼───┨
┃   │   │   ┃┃   │   │   ┃┃   │ o │   ┃┃   │   │   ┃┃   │   │ o ┃┃   │   │   ┃
┠───┼───┼───┨┠───┼───┼───┨┠───┼───┼───┨┠───┼───┼───┨┠───┼───┼───┨┠───┼───┼───┨
┃   │   │   ┃┃   │ o │   ┃┃   │   │   ┃┃ o │   │   ┃┃   │   │   ┃┃   │   │   ┃
┗━━━┷━━━┷━━━┛┗━━━┷━━━┷━━━┛┗━━━┷━━━┷━━━┛┗━━━┷━━━┷━━━┛┗━━━┷━━━┷━━━┛┗━━━┷━━━┷━━━┛
     →XM          →YL          →LL          →YN          →LM          →??

Adri

Done (unless there's a hidden catch!) :-)

Adri - How long did it take you to put the grid together so that it looks right??? :-)

Cheers - Al.