In: Biology
We know that we have two sides of the brain,
the left and the right
The left side of the brain is involved in the process of active learning, processing of new information, logics, processing mathematical and scientific skills.
While
The right side of the brain is involved in creative fields such as art, music, imagination, rhythm, visualisation etc
When an activity is learnt in the due course of time like the one mentioned above (guitar which the boy has been playing for 10 years ), there is no active learning or logistics that is involved into dooing that activity, hence the whole act was predominantly coordinated by the creative or right side of the brain,
But when the same boy was reading or studyig something for the philosophy exam, the process of active learning was involved and the left side of the brain was very active.
When the mother tried to tell something to the boy at both these instances, the boy remembered or could process the new information because there was a place ion the brain for processing of the new informatoion, since the playing of the guitar was done predominantly by the right side of the brain,
while in the second scenario, the left side of the brain was already occupied in the process of active learning, hence no further information could be processed.
(By the above statements, i do not mean to say that at all times, the brain has to perform a particular activity by one hemisphere only, usually both the halves of the brain work together for carrying out an activity, yet, they show predominance to certain activities)
I hope i was able to help you with the answer !:)