When all numbers from 1 though 19
are written in a row, the two numbers which are removed from the
opposite sides of the row (one from each side) always total 20, i.e. 1
and 19, 2 and 18, and so on. Each of such pairs should be placed on
the opposite sides of any straight line shown in the illustration.
After all nine pairs are placed, the remaining number 10 goes into the
central circle making the numbers in every 3 circles on a straight
line total 30.