Lesson plan Grade 3 Math
Sparrow Hill's Monthly Water Log: Adding and Subtracting to 1000
A grade 3 lesson where students use a fictional school garden's monthly water-use log to add and subtract three-digit gallon totals within 1000.
Objectives
- Students will subtract three-digit numbers within 1000 to find the change in the school garden's water use from one month to the next.
- Students will add two three-digit numbers within 1000 to find a combined total.
- Students will explain what an increase or decrease in the log means about the garden's water use that month.
Vocabulary
Key words: place value, regroup, difference, total
Teacher overview
The fictional Sparrow Hill School keeps a water conservation log posted outside the office, showing how many gallons the school garden's sprinklers used each month: 845 in September, 690 in October, 512 in November, and 398 in December. Students use this log to practice fluent three-digit addition and subtraction, always staying within 1000, while also reading the numbers as a record of the garden using less water as the year went on. A common misconception in this lesson is that students apply regrouping by habit, borrowing on every subtraction or carrying on every addition, instead of checking whether a given column actually calls for it. Each subtraction drawn from the log requires a borrow, while the rain-barrel addition (215 + 340) requires no carrying, so students have to read the columns rather than assume.
Procedure
Hook (5 min): Show the water log on the board with all four monthly totals. Ask students: did the garden's sprinklers use more or less water in October than in September? How could we find out exactly how much the amount changed?
Direct instruction (10 min): Model finding the difference between September's and October's totals using a place-value strategy or the standard algorithm: 845 − 690 = 155, so the sprinklers used 155 fewer gallons in October. Show the regrouping step clearly on the board, borrowing from the hundreds place to complete the tens column. Point out that this problem required regrouping, and ask students to predict whether the next comparison will too.
Guided practice (15 min): In pairs, students find the change from October to November: 690 − 512 = 178 fewer gallons. Circulate and check that students align place values correctly and regroup where needed. Then introduce a second kind of problem: two rain barrels were emptied and refilled all through November; one collected 215 gallons in all and the other collected 340 gallons in all. Pairs add the two totals to find how much the rain barrels collected combined: 215 + 340 = 555 gallons.
Independent practice (10 min): Each student solves two problems on the handout: (1) find the change in water use from November to December (512 − 398), and (2) find the total change in the garden's water use from September all the way to December (845 − 398).
Closure / exit ticket (5 min): Students answer one question on an index card: a classroom rain barrel that was emptied and refilled during December collected an extra 125 gallons in all, which was not part of the garden's log. If that rain barrel total is added to December's logged amount of 398 gallons, what is the new combined total? Collect the cards and check whether students set up the addition correctly.
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: 114 gallons.
512 − 398 = 114 fewer gallons were used in December than in November.Answer: 447 gallons.
845 − 398 = 447 fewer gallons were used in December than in September, the total change across the whole period shown in the log.Exit ticket answer: 523 gallons.
Adding the rain barrel's 125 gallons to December's logged total of 398 gallons: 398 + 125 = 523 gallons.
Differentiation
Support (below grade level)
Provide a place-value chart with hundreds, tens, and ones columns already drawn, and allow base-ten blocks for the regrouping steps. Start with the rain-barrel addition that needs no carrying (215 + 340) before moving to the subtraction problems, which each require a borrow.
Extension (above grade level)
Ask students to find the garden's total water use across all four months added together (845 + 690 + 512 + 398), which exceeds 1000, and explain in writing what strategy they used to add four three-digit numbers when the running total goes past 1000.
ELL supports
Pre-teach "increase," "decrease," and "total" with simple up-arrow and down-arrow icons next to the log's monthly numbers. Provide a sentence frame for the closure discussion: "The sprinklers used ___ gallons more/fewer in ___ than in ___."
Extensions
Have students design a simple bar or line graph of the four monthly totals from the log, then write one sentence describing the overall trend across the school year, connecting this lesson's addition and subtraction practice to graphing skills used elsewhere in the grade.