Problem
Add the two readings on either side of 1 p.m.: 11 + 15 = 26.
The connecting line climbs evenly, so the middle is the average of the ends.
5.MD.B.2Look For A PatternDivide the sum by 2 to find the midpoint value: 26 ÷ 2 = 13 degrees C.
On the graph 13 sits exactly halfway up the segment, matching 1 p.m. halfway across.
5.MD.B.2Draw A DiagramDividing the two readings' total, 26, by 2 lands on the temperature that sits exactly halfway between 11 and 15.
Halfway means the value rises the same amount above 11 as it drops below 15, so 11 is that middle value lowered by one step and 15 is the same middle value raised by that same step.
Adding 11 and 15 then adds the middle value lowered by a step to the middle value raised by an equal step, and lowering then raising by the same-size step cancels out.
Taking a step down and then adding back a step of the same size returns the amount to where it started, so the two steps undo each other.
With the two steps cancelled, the sum 11 + 15 is just the middle value counted twice, which is 2 equal groups of that middle value.
Since the total 26 is 2 equal groups of the middle value, dividing it by 2 undoes that doubling and hands back the single middle value.
Halfway across a line segment means halfway up - just average the two end values!