First Grade · Math · week 8

Subtract by thinking addition

Solve a subtraction fact by thinking of the related addition fact.

Here is a fast way to subtract: turn it into an addition question. For 13 - 8, ask yourself "8 plus WHAT makes 13?" You already know 8 + 5 = 13, so 13 - 8 = 5.

Every subtraction fact hides an addition fact. This strategy is best for bigger numbers, where counting back would take too many hops.

📋 For the teacher
Teach it (10–15 min):
  • Rewrite every subtraction as "___ plus what?" out loud before solving.
  • This is the moment addition and subtraction click together as one family -- point that out.
  • Use this strategy especially when the number being subtracted is 6 or more.
  • Let your child choose count-back or think-addition -- whichever is faster for that problem is correct.
Talk about it:
  • What addition question does 15 - 8 turn into?
  • Why is think-addition better than counting back for 16 - 7?
  • How are addition and subtraction connected?

Mastery looks like: They solve subtraction facts by recalling the related addition fact instead of counting.

Common stumbles: Not knowing the addition fact well enough to reverse it; reaching for slow counting when a fact is already known.

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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