Y=1/2x represents a Linear equation from the form y = mx + b (In our case, b = 0 so we don't put it). In such a form, m represents the slope of the equation. And b represents the y-intercept of the equation. While y and x represents the coordinates of 2 points. In y=1/2x, m=1/2; b = 0. So the coordinates are y and x. I've added a graph of the equation y= 1/2x under the answer.
The 2 in Jacobs amount represents 20 because it is in the tens place. While the 2 in Barbaras amount represents 200 because it is in the hundreds place.