There was a good example of a tricky 'not given' answer in last week's reading exercise. Let's look at question 2 again.
The optimal amount of physical activity for children is 10 minutes.
Relevant part of the passage:
short bursts of physical activity - 10 minutes or less - increased pupils’ attention in the classroom
Why is the answer 'not given'? Well, the key word in the question is 'optimal' - it might help if we replace this word with the word 'best'. Does the passage tell us that 10 minutes is the best amount of physical activity? No, we only know that 10 minutes or less is enough exercise to increase pupils' attention. Maybe 15 minutes works even better; we don't know.
Sometimes it is just one key word that makes the answer 'not given'.