First Grade · Math · week 13

Count to 120 crossing the decades

Count forward to 120, including across decade boundaries such as 29 to 30 and 99 to 100.

The tricky part of counting is crossing from one decade to the next, like going from 29 to 30, or 49 to 50. The ones reset to 0 and the tens go up by one.

The biggest jump is 99 to 100 -- a whole new set of digits appears! After 100 comes 101, 102, and on up to 120.

📋 For the teacher
Teach it (10–15 min):
  • Practice counting out loud daily, especially through the tricky decade crossings, like 29-30, 59-60, and 99-100.
  • Use a hundred chart, and past 100 tape on an extra strip so 101-120 has a visible home.
  • Slow way down right at each decade boundary -- that is where mistakes happen.
  • Celebrate reaching 100 -- it is a big milestone worth making a fuss over.
Talk about it:
  • What number comes right after 49?
  • What is tricky about going from 99 to 100?
  • Can you count from 90 all the way to 105?

Mastery looks like: They count forward to 120 without hesitating at a decade crossing or at the century mark.

Common stumbles: Saying "twenty-ten" instead of "thirty"; freezing or guessing at 99 to 100.

See it visually

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

Counting & Ten-FramesCount objects in ten-frames and base-ten blocks, one tap at a time.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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