漫画解读
观察上面温画，结合所学知识，回答下列问题：

读短文，并判断正误。

                                                                       Summer Holidays(暑假) in England

    With July coming, the summer holidays begin. Boys and girls will have 60 days to rest. The weather is good. Parents will take their children to the seaside(海边) for the holidays. The children like the sand(沙子), the sun, the cool wind and the water. Certainly there are lots of things to see, good things to eat, and exciting things to do. They can play ball games. They can go on field trips, too. They study real things on the field trips. What good holidays!

在一个不透明的布袋中，红色、黑色、白色的玻璃球共有40个，除颜色外其他完全相同．小张通过多次摸球试验后发现，其中摸到红色、黑色球的频率稳定在15%和45%，则可估计袋中白色球的个数是____