Children In A Circle
A group of children stand holding hands in a large circle and a teacher walks around the circle giving each child in order a number 1, 2, 3, 4, ...
If number 12 is standing opposite number 30, how many children are there in the circle?
Let c be the number of children in the circle. Half the children, c/2, will be in-between child 12 and child 30.
So 12 + c/2 = 30.
Therefore c/2 = 18.
That is, there are 36 children in the circle.