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Cogito

Grade 1 Math · Chapter 5 · Lesson 8

Answer Key

Missing Numbers to Twenty

The box can be anywhere in the sentence.

10 problems · about 13 minutes · 1.OA.D.8

What this lesson teaches

The child finds the unknown in equations within 20 with the unknown in any position.

  • The missing number can be anywhere.
  • Start from what you already know.
  • Make a ten on the way if it helps.

Warm-Up Review

From earlier lessons. Loosen up before the new work.

4 problems

  1. Review — Take From Ten in Stories: 11 − 8 = ?

    Answer 3

    Why 11 − 8 = 2 + 1 = 3.

  2. Review — Taking From Ten: 13 − 9 = ?

    Answer 4

    Why 13 − 9 = 1 + 3 = 4.

  3. Review — Counting On or Making Ten?: 8 + 2 = ?

    Answer 10

    Why 8 + 2 = 10.

  4. Review — Start With the Bigger One: 6 + 9 = ?

    Answer 15

    Why 9 + 6 = 10 + 5 = 15.

Warm Up

Straightforward practice. Get the method working first.

3 problems

  1. 8 + ? = 12

    Answer 4

    Why 8 + 4 = 12.

  2. 13 − ? = 9

    Answer 4

    Why 13 − 4 = 9.

  3. 9 + ? = 13

    Answer 4

    Why 9 to 10 is 1, then 10 to 13 is 3.

Build It Up

The same ideas with more to keep track of.

1 problem

  1. ? + 6 = 14

    Answer 8

    Why Count up from 6 to 14.

Stretch Yourself

Mixed problems. Work out what kind of question it is before you start.

2 problems

  1. 15 − ? = 8

    Answer 7

    Why Count up from 8 to 15.

  2. Saving Up: Ben has 8 coins. He needs 13. How many more does he need?

    Answer 5 coins

    Why 8 + 5 = 13.