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+### Statement
+
+$9.2.21.$ [Insert the problem statement]
+
+### Solution
+
+### Statement
+
+$9.2.21.$ Estimate the magnetic field induction at the center of a flat iron ring of thickness $1$ cm with an internal radius of $10$ cm and an external radius of $20$ cm. All iron atoms are oriented along the axis of the ring, the magnetic moment of an iron atom is $2\mu_e = 1.85\cdot10^{-23}$ J/T.
+
+### Solution
+
+We will use the solution to the previous problem (see [9.2.20](https://savchenkosolutions.com/en/9.2.20)). The field at short distances can be expressed through the result of the previous problem as:
+
+$$
+B = B_{R_1} - B_{R_2} = \frac{\mu_0 M h}{2} \left( \frac{1}{R_1}-\frac{1}{R_2} \right)
+$$
+
+Let's estimate the magnetic moment per unit volume (magnetization):
+
+$$
+M = \frac{2\rho N_A \mu_e}{M_{Fe}}
+$$
+
+Substituting and calculating:
+
+$$
+B = \frac{\mu_0\rho N_A \mu_eh}{M_{Fe}} \left( \frac{1}{R_1}-\frac{1}{R_2} \right) \approx 4.9 \cdot 10^{-2} \text{ T}
+$$
+
+#### Answer
+
+$$
+B = \frac{\mu_0\rho N_A \mu_eh}{M_{Fe}} \left( \frac{1}{R_1}-\frac{1}{R_2} \right) \approx 4.9 \cdot 10^{-2} \text{ T}
+$$
+
+#### Answer
+
+[Insert a concise answer or boxed result]