Second Grade · Math · week 17

Mental math: plus and minus 10 and 100

Add or subtract 10 or 100 mentally from any number by changing only the tens or hundreds digit.

Adding or subtracting 10 is a mental math shortcut: only the tens digit changes. 253 + 10 = 263 -- the 2 (hundreds) and 3 (ones) stay exactly the same.

Adding or subtracting 100 works the same way for the hundreds digit. 253 + 100 = 353 -- only the hundreds digit moves.

📋 For the teacher
Teach it (10–15 min):
  • Push them to answer instantly, without writing anything down -- this is meant to become an in-your-head skill.
  • If a tens digit is 9 (like 195 + 10), notice the tens digit rolls to 0 and the hundreds digit goes up -- that is the one tricky case.
  • Same watch-out for hundreds: if the hundreds digit is 9, it rolls over into a new thousand -- mention it but do not dwell; it is rare at this stage.
  • Play a quick back-and-forth game: you say a number, they say the plus-10 and minus-10 answers immediately.
Talk about it:
  • Which digit changes when you add or subtract 10? Which digit changes for 100?
  • What happens when the tens digit is already a 9 and you add 10?
  • Can you do 528 + 10 and 528 - 10 both in your head?

Mastery looks like: They add or subtract 10 or 100 from any 3-digit number mentally and instantly, including numbers that roll over a digit.

Common stumbles: Changing the wrong digit (adjusting ones instead of tens); needing to write out the whole problem instead of doing it mentally.

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 →Place ValueWatch ones, tens, hundreds, and thousands build big numbers.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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