Problem 4. The sum of the numbers in each of the four given rings is 90. Which numbers are hidden at the places marked with the letters $A, B, C$ and $D$?
Solution. From the right ring, we get

$C=90-(7+9+11+39+14)=10$ From the bottom ring, we get

![](https://cdn.mathpix.com/cropped/2024_06_05_bbda9f5c2f069cdaaac4g-01.jpg?height=507&width=695&top_left_y=1669&top_left_x=806)
that

$$
B=90-(10+14+31+23+10)=2
$$

From the left ring, we get

$$
D=90-(2+10+30+16+15)=17
$$

From the top ring, we get

$$
A=90-(1+7+10+2+15)=55
$$