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I am struggling with those two possible meanings, and have found two examples
that typically puzzle me:

  1. 「この問題できる？」「こんなのわけないよ」

  2. 「N2の試験に合格したんだって？」「そんなわけないだろう」

(taken from Nihongo So-Matome N2 book)

First one is given as holding the "easy" meaning, second one, the "no
way/impossible" meaning... although for me their writing here seem _almost_
identical.

I can see that the first one has this の inside, and that probably impacts the
meaning! But can someone develop around it, and tell me what is going on here
that makes the meaning differ ?

I've [found
somewhere](https://learnjapanesebod.tumblr.com/post/157848565348/%E3%82%8F%E3%81%91-the-
word-%E8%A8%B3-%E3%82%8F%E3%81%91-translates-to-meaning) that the "easy"
meaning can be deciphered when the expression is used as an adjective, which
concurs with Jisho too. But here, it is not as far as I can understand.

Maybe also on a more fundamental level, how such an expression (わけ（が・は）ない）can
have two such opposite meaning ?
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question
meaning usage
わけない dual meanings: impossible vs easy
The real meaning of wakenai is "no understanding" or "no reason", so I'd
translate your sentences as:

> 「この問題できる？」「こんなのわけないよ」  
>  Can you do this question? You asking me makes no sense, of course.

With the "yo" translating as "of course".

The second sentence:

> 「N2の試験に合格したんだって？」「そんなわけないだろう」  
>  Did you pass the N2 exam? That thing makes no sense to me.

So the meaning of "wakenai" differs based on context.
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answer
What you see is a result of two subtle (but basic) grammar points combined.

  1. こんなの/そんなの vs こんな/そんな

こんな/そんな… are (technically not adjectives, but) a kind of single function
adjectives, that are only allowed to qualify a noun.

の has tons of meanings, among which is nominalization. When の comes after 連体形
(form used to qualify a noun) of adjectives or verbs, it can make a noun
phrase e.g. きれいなの "beautiful one", 走るの "something that runs". Thus こんなの/そんなの…
are nouns as a whole.

  2. わけ (noun) vs わけ (形式名詞 "formal noun")

わけ has multiple meanings as a noun, but aside from that, it is most often used
as a 形式名詞, which grammatically behaves as if a noun, but always needs a
qualifier and serves as a function word in meaning.

Now, わけ + ない creates two idiomatic expressions:

  * **わけ (noun)** (+ optional particles) + **ない**  
"no reasoning needed" = "is no-brainer"

  * **X わけ (形式名詞)** (+ optional particles) + **ない**  
"there is no way X", "it cannot be X"

> こんなのわけないよ  
>  そんなわけないだろう

The first sentence starts from こんなの, which is a noun, impossible to qualify
the next わけ. So this sentence can be only parsed as "こんなの [omitted topic
particle] わけない".

The second sentence starts from そんな, and it **must** qualify the next わけ, thus
this わけ is a 形式名詞. The sentence is parsed as "そんなわけ [omitted particle] ないだろう".
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answer