10 More or 10 Less
Mentally find 10 more or 10 less than a given two-digit number without counting
What a learner can do afterwards
- Given 56, quickly say 66 is 10 more and 46 is 10 less
- Explain that adding 10 increases the tens digit by 1
- Answer '10 more than 73' without using fingers or counting on
The lesson
Every two-digit number has a tens digit and a ones digit. When you add 10, the tens digit goes up by one. The ones digit never changes. Taking away 10 works the same way, but the tens digit goes down by one.
Ravi has 34 marbles. His uncle gives him 10 more. Ravi does not count one by one. He looks at the tens digit, 3, and makes it 4. The ones digit 4 stays the same. Ravi now has 44 marbles.
To find 10 more or 10 less fast, cover the ones digit with your finger. Change only the tens digit. Then uncover the ones digit. It never moved.
10 more or 10 less only changes the tens digit; the ones digit always stays the same.
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.