15. Each letter in the figure represents a number, and the sum of the three numbers in each circle is 21, and the sum of all the numbers is 69. Then $b=$ $\qquad$ .
Answer: 10
Explanation: The sum of the three numbers in each circle is $21$, and the sum of the 5 circles is $21 \times 5=105$. In 105, five numbers are repeated (2, 8, 9, b, d). Subtracting these five repeated numbers from 105 gives the total sum of all numbers. $105-(2+8+9+b+d)=69, b+d=17$. From $d+5+9=21$ we get $d=7$, so $b$ equals $17-7=10$.