2 1+ 10=3 1,23+ 10=33;

3 1+33=64

Fill in 64 at the top of the first row of circles.

The second row of circles is filled with 3 1 and 33 from left to right respectively.

The third row of circles is filled with 2 1 and 23 from left to right respectively.

This is an addition problem. The numbers in the two circles add up to get the number in the last circle.