From Oxford dictionary I have the first sentence. But from my grammar book is the following quote: "When the future event doesn't depend on the action described in the if-clause, we use be going to, not will." So, is it not better to use "will" in this example because there is a need for a clue?
I'm never going to guess the answer if you don't give me a clue.
I will never guess the answer if you don't give me a clue.
Generally speaking, be going to is the standard future tense of English. Almost everywhere you can use "will", or present continuous or simple present with future meaning, you could use be going to instead. Another way of thinking about this is, "To choose a future verb other than be going to, you need a good reason."
There are several future contexts where "will" is correct, but that doesn't mean only "will" is correct. Often, be going to is also correct.
This is one of the situations where be going to is also correct.
It's probably more common for people to use "will" in a context like this, but only because it's shorter. Both sound 100% natural.
