Magnetic field inside a sperical coil

[adamoob]

Homework Statement

What is the induction in the centre of a spherical coil with N turns, the current I and the radius R?

Homework Equations

Biot-Savart law \mathbf{B} = \int\frac{\mu_0}{4\pi} \frac{I d\mathbf{l} \times \mathbf{\hat r}}{|r|^2}

The Attempt at a Solution

I know what the induction from one turn is but I don't have a clue how to sum these up. And as have to have this homework done by Tuesday I'll be most grateful for any help.

 

[zzzoak]

In Biot-Savart law you may change
I to NI.