Two-digit numbers: by digits, by parts, the rounding trick
Add and subtract two-digit numbers by digits and by parts, and work out 38 + 9 through 10, giving back the extra one. Section: Grade 2; Grade 3 oral methods are in another studio.
- France: grade 1
- Germany: grade 2
- Ukraine: grade 1
- Russia: grade 2
Guide
In the Grade 2 book (NUSH) two-digit numbers are first added digit by digit: tens with tens, ones with ones. 46 + 23: 40 + 20 = 60, 6 + 3 = 9, altogether 69. If there are more than 9 ones, the extra ten moves to the tens.
Then children work by parts: to 74 add the tens of the second number, then its ones: 74 + 23 = 74 + 20 + 3 = 94 + 3 = 97. Subtracting is the same: 74 − 23 = 74 − 20 − 3. The same sum can be done in different ways and the ways compared.
The rounding trick helps when the second number is a little less than whole tens. 38 + 9: instead of 9 add 10, then take away 1. 38 + 9 = 38 + 10 − 1 = 47. And 45 − 19 = 45 − 20 + 1 = 26: we took away 1 too many, so we give 1 back.