Mental addition and subtraction (age 9+)
Add and subtract numbers mentally with increasingly large numbers, using place-value knowledge and derived facts
What a learner can do afterwards
- Mentally calculate 45,000 + 8,000
- Mentally subtract 3,200 from 10,000 by counting up
- Use near-doubles: 2,500 + 2,600 = 5,100
The lesson
You already know how to break numbers apart by place value and use facts you know to find new ones. Now you use those same tricks on bigger numbers, like thousands.
Kai adds 45,000 and 8,000. He knows 45,000 is 45 thousand and 8,000 is 8 thousand. He just adds the thousands: 45 + 8 = 53. So 45,000 + 8,000 = 53,000.
Priya adds 2,500 and 2,600. She knows the double 2,500 + 2,500 = 5,000. Since 2,600 is 100 more than 2,500, she adds that extra 100 too: 5,000 + 100 = 5,100.
Look for the round numbers hiding inside a big number. Breaking it into thousands, hundreds, or a double you already know makes the whole problem easier.
Break big numbers into place value chunks, count up to subtract, and use near-doubles to add fast in your head.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.