Problem
Class 3 has one more circle than Class 2 but only 4 more students, so that one circle stands for 4 students.
Matching the known student-difference to the circle-difference reveals the scale.
3.MD.B.3Analyze The UnitsOn this graph, one circle stands for 4 students.
Class 3 is drawn with one more circle than Class 2, yet in real students Class 3 has just 4 more than Class 2, so that single extra circle is worth exactly those 4 students.
Class 3's students split into 'as many as Class 2' plus the students shown by the one extra circle, so that extra circle carries the whole 4-student gap between them.
Every circle in a column stands for the same number of students, so if one circle is worth 4 then each circle is worth 4.
At 4 students per circle, Class 2 = 3 × 4 = 12 students and Class 3 = 4 × 4 = 16 students (16 - 12 = 4, as required).
Multiplying circles by the scale converts the drawing to real counts.
3.MD.B.3Analyze The UnitsThe four classes total 56. Subtract the three known classes: 56 - 16 - 12 - 16 = 12, so Class 4 has 12 students.
Total minus the known parts gives the missing part.
3.OA.A.3Identify SubproblemsDivide each count by 4: Class 1 = 4 circles, Class 2 = 3, Class 3 = 4, Class 4 = 3 circles; draw each column from the bottom up.
Dividing each count by the scale tells how many circles to draw in each column.
3.MD.B.3Identify SubproblemsThis only needs the Grade 3 idea that one circle's difference equals the student difference — that sets the scale!