Lesson plan Grade 4 Math

Thornbury's Rain Gauge Study: A Line Plot and Fraction Lesson

A grade 4 lesson where a fictional class plots two weeks of rain gauge readings in eighths of an inch, then adds and subtracts fractions to find totals and differences.

Grade: Grade 4
Subject: Math
Material: Lesson plan
Time: 50 min
Prep: Low prep
Materials: Printed rain gauge data table, Blank number line handout marked in eighths from 0 to 1 inch, A rain gauge or a picture of one, Index cards for the exit ticket

Standards

4.MD.B.4 Common Core Math

Make a line plot to display a data set of measurements in fractions of a unit (1/2, 1/4, 1/8). Solve problems involving addition and subtraction of fractions by using information presented in line plots.

How this material aligns: Students plot eight rainfall measurements recorded to the nearest eighth of an inch on a class line plot and then read the completed plot to answer addition and subtraction questions, precisely the skill this standard describes.

All materials for 4.MD.B.4 →

4.NF.B.3 Common Core Math

Understand a fraction a/b with a > 1 as a sum of fractions 1/b, and add and subtract mixed numbers with like denominators; solve word problems involving addition and subtraction of fractions referring to the same whole and having like denominators.

How this material aligns: Finding the two-week rainfall total and the gap between the wettest and driest days both require adding and subtracting eighths that share the same denominator directly from the line-plot data, the fraction work this standard covers.

All materials for 4.NF.B.3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-16.

Objectives

Students will record measurement data to the nearest eighth of an inch on a line plot.
Students will add and subtract fractions with the same denominator using data read from the line plot.

Vocabulary

Key words: line plot, eighth, like denominator, rain gauge

Teacher overview

A fourth-grade class in the fictional town of Thornbury keeps a rain gauge outside their classroom window and records how much rain falls after each storm, measuring to the nearest eighth of an inch. Over two school weeks, the class collects eight measurements and uses them to build a line plot, then reads that line plot to find the total rainfall for the period and the difference between the wettest and driest days. A common misconception in this lesson is that students add or subtract the number of tally marks on the line plot instead of the fraction values those marks represent, confusing "how many times it rained an eighth of an inch" with "how many total eighths of an inch fell."

Procedure

Hook (5 min): Show a picture of a rain gauge (or a real one, if available) and ask students how someone could measure rain precisely enough to know it fell "three-eighths of an inch" rather than just "a little" or "a lot." Explain that Thornbury's class rain gauge is marked in eighths of an inch for exactly this reason.

Direct instruction (10 min): Present the class data table: eight storm measurements in eighths of an inch were 1/8, 2/8, 1/8, 3/8, 4/8, 1/8, 3/8, and 2/8. Model plotting the first four values as X marks above a number line marked in eighths from 0 to 1. Show that the line plot lets you see at a glance which measurement happened most often (1/8 inch, three times).

Guided practice (15 min): As a class, finish plotting the remaining four measurements together. Then model finding the sum of just the first four values plotted: 1/8 + 2/8 + 1/8 + 3/8 = 7/8 inch, adding the numerators since every denominator is 8. Ask students to explain in their own words why the denominator stays 8 throughout the addition.

Independent practice (10 min): Using the completed line plot, students find two things on the handout: (a) the total rainfall for all eight measurements, and (b) the difference in depth between the wettest day (4/8 inch) and the driest nonzero day (1/8 inch).

Closure / exit ticket (10 min): On an index card, students answer: if it rains an additional 1/8 inch on a ninth day, what would the new two-week total be? Collect the cards and check that students added the new eighth to their independent-practice total rather than starting over.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answer: 2 1/8 inches.
Independent practice (a), total rainfall: all eight values in eighths are 1, 2, 1, 3, 4, 1, 3, 2, which sum to 17 eighths. 17/8 = 2 1/8 inches. (The guided-practice partial sum of the first four values, 7/8 inch, is modeled in the procedure.)
Answer: 3/8 inch.
Independent practice (b), difference: the wettest day (4/8 inch) minus the driest nonzero day (1/8 inch) equals 4/8 − 1/8 = 3/8 inch.
Answer: 2 1/4 inches.
Exit ticket: adding one more 1/8-inch day to the 17/8-inch total gives 17/8 + 1/8 = 18/8 inches, which simplifies to 2 2/8, or 2 1/4 inches.

Differentiation

Support (below grade level)

Provide a partially completed line plot with four of the eight measurements already marked, so students only add the remaining four themselves. Allow students to use fraction strips or a number line marked in eighths as a physical tool for addition and subtraction instead of working with numerators alone.

Extension (above grade level)

Ask students how many more eighths of an inch would have to fall for the two-week total to reach 3 inches, and to show the subtraction in eighths (24/8 − 17/8 = 7/8 inch) along with how they knew that 3 inches equals 24 eighths.

ELL supports

Pre-teach "gauge," "measurement," and "denominator" using a labeled picture of a rain gauge marked in eighths, pointing to the same fraction on both a ruler-style gauge and the number line handout. Provide a sentence frame: "The two-week total is ___ inches because ___ plus ___ plus ___ (and so on) equals ___ eighths."