Mental math on ten more and ten less, anchored by a callout: only the tens digit changes, the ones digit stays exactly the same. Students complete a chart finding both neighbors of 24, 58, 31, 76, 40 and 89, answer quick lines like '10 more than 43 is ______', then explain in writing why Leo's shortcut for 10 more than 62 works.
1.NBT.5
“Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.”
The standard asks for the answer without counting and for the reasoning to be explained. Part 1 supplies the first; Part 3 demands the second in the student's own words.
A three-column chart runs 10 Less, Start Number, 10 More for six numbers: 24, 58, 31, 76, 40 and 89. The instruction says to find the answers mentally.
Single-line items such as '10 more than 43 is ______' and '10 less than 67 is ______', to be answered without counting by ones.
Question 7 quotes Leo: to find 10 more than 62 he changes the 6 tens to 7 tens and gets 72. Students write why the rule works and what happens to each digit.
Hundred chart beside it
Let students verify two chart rows by jumping a row on the hundred chart, then take it away. The point is the pattern, not the tool.
40 is the interesting row
Ten less than 40 is 30, and a few students will write 3 or 39. That row shows who is operating on digits without meaning.
Read the Part 3 answers
Accept any wording that says the tens digit moves by one and the ones digit is untouched. That sentence is the whole standard.
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