why girls are not as good as boys in maths?

no i dont think so .

There is no difference in the performance of men & women when it comes to math. Where I went to High School there were more females in advanced math than males. Here is an article about the cognitive differences in men & women: http://www.psychologymatters.org/thinkagain.html

The reason women have more trouble with math is culture based. Not gender based. In most cultures, women are taught to think socially, about keeping family and friends happy and together. This requires more emotional thinking rather than logical thinking. Men are taught to think logically and solve problems without emotion. Women can certainly overcome this cultual teaching and do very well at math.