Problem
The filled dot counts its number while the open circle at 25 does not, so the run ends at 24.
A solid dot 'grabs' its number; an open circle 'lets go' of its number. Sixth graders read these exact symbols on a number line.
6.EE.B.8Draw A DiagramTrying 16 lists 16 through 24, which is exactly 9 numbers.
Listing the actual numbers on the line and counting them is a safe way to check a guess instead of hoping it is right.
6.NS.C.6Make A Systematic ListThe count is (24 - start) + 1 = 9, so 25 - start = 9 and the start is 16.
The number of whole numbers from one mark to another is the gap plus one, so the count pins down the start exactly.
6.EE.B.8Guess And CheckThe count of whole numbers from the left end up to 24 is the gap 24 minus that end, plus one.
Subtracting the two ends counts the steps between them, not the numbers sitting at them.
The run holds one more number than it has steps, because the starting number is already there before a single step is taken.
The run is the first number together with one new number for each step, with none missed and none counted twice.
A solid dot counts its number, an open circle doesn't — so just list the numbers that stay inside and count them.