Even though it is true that most of the brain is involved in complex tasks, there is some research to suggest that there are specific areas that are more important for problem solving or maths. This is indicated in core deficits in certain individuals. Those are people with low performance in specific areas compared to other areas (what's called a dissociation). However, the number of people that show such a deficit is rare.
Brian Butterworth argued that such a core deficit exists for Maths ability. People with this true dyscalculia show a deficit even in very simple comparisons of different numbers of dots, but are in the typical range for other abilities, e.g. verbal reasoning. There is a network of brain regions that corresponds to this, comprising the angular gyrus, the superior parietal lobule, and the intra-parietal sulcus.
There is an excellent introduction to these topics (also including a discussion of general reasoning and the brain) in this book:
Mareshal, Butterworth (editors) (2013): 'Educational Neuroscience', 1st edition, Wiley-Blackwell