The number of children is 4 and the number of notebooks is 60.
<u><em>Explanation</em></u>
Lets assume, Mama has
number of children and
number of notebooks.
If she give 13 notebooks to each of her children, then 8 books are left. So, 
And If she give 15 notebooks to each of her children, she will have nothing left. That means 
Now solving the system of equations, we will get ....

Now, 
So, the number of children is 4 and the number of notebooks is 60.