Seventh Grade · Math · week 10

Adding and subtracting negative numbers

Add and subtract signed numbers using number-line and chip (zero-pair) models, and understand subtraction as adding the opposite.

On a number line, adding a positive number moves you RIGHT, and adding a negative number moves you LEFT. So -3 + 5 starts at -3 and moves 5 steps right, landing on 2.

A chip model works too: positive chips and negative chips cancel each other out in pairs (a "zero pair"), since 1 + (-1) = 0. Whatever chips are left over are your answer.

Subtraction has one big rule: subtracting a number is the exact same as adding its opposite. So 4 - 7 becomes 4 + (-7), and -2 - (-7) becomes -2 + 7. Once you rewrite subtraction as adding the opposite, every problem turns back into the addition rules above.

📋 For the teacher
Teach it (10–15 min):
  • Draw an actual number line on paper and physically trace each hop with a finger, left for negative, right for positive.
  • Use two colors of real chips or coins (one color positive, one negative) and physically remove zero pairs.
  • Repeat the rule out loud every time subtraction shows up: "subtracting a number means adding its opposite."
  • Start with smaller numbers and check answers by "walking back" the hops in reverse.
Talk about it:
  • Why does adding a negative number move you LEFT on the number line?
  • What does a "zero pair" mean in the chip model?
  • Why does -2 - (-7) turn into an addition problem?

Mastery looks like: They add and subtract signed numbers accurately using either model, and can explain subtraction as adding the opposite.

Common stumbles: Forgetting to flip a double negative into addition; moving the wrong direction on the number line for a negative number.

See it visually

Interactive, spoken-aloud tools that show this idea in motion.

SubtractionTake away and count back, step by step.try →AdditionSee groups combine — with counters, blocks, and the number line.try →
Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Math for this learner.
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