A 13-year-old boy took less than a second to answer this maths problem

[TightEnd]

answers please

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[RED-DOG]

answers please

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I answered it in less than a second too.

[hector62]

I am going for 25% of them.

[Doobs]

I am going for 25% of them.

I am with Hector.  More than a second.  Think it is pretty tricky to do in a second unless you have seen it previously.  First thought is always 50%.

[TightEnd]

a chick can get pecked in there ways: from R, L, or both. each event has probability of (1/2)*(1/2)=1/4. total probability of being pecked is thus 3/4. so 1-3/4 =1/4 prob of not picked.

25 of 100 unpecked

https://www.nytimes.com/2017/05/15/us/math-counts-national-competition.html?smid=tw-nytimes&smtyp=cur&_r=0

[Doobs]

a chick can get pecked in there ways: from R, L, or both. each event has probability of (1/2)*(1/2)=1/4. total probability of being pecked is thus 3/4. so 1-3/4 =1/4 prob of not picked.

25 of 100 unpecked

https://www.nytimes.com/2017/05/15/us/math-counts-national-competition.html?smid=tw-nytimes&smtyp=cur&_r=0

Love this comment.

The contestants' achievements were the results of endless hours of practice and not necessarily innate math (sic) abilities.

Have you read "thinking, fast and slow", Tighty.  I have just started reading it, but I think it is going to be very good.  Very much like "Fooled by Randomness".  Anyway this looks a classic example.  Your instinct tells you it is 50%, but you really need to step back and think the problem through.  Unless it was in your practice questions anyway!