Redo the problem, or check it the other way: undo a sum by subtracting, a difference by adding. Say what each digit is worth to explain why a step works.
checkmake sure an answer is right
strategya way of solving
carrymove a ten into the tens column
inversethe opposite operation that undoes it
Watch for: Calling an answer right without working it out. Missing the forgotten carry: 46 + 18 written as 54. Thinking the carried 1 is worth one instead of ten.
At home: Make a deliberate mistake on a sum and ask your child to find it and explain what went wrong.
Sample sheet and answer key
Freddie's Frog Pond
Grade 2 · Checking strategies · 2.NBT.B.9
NameDate
Freddie the frog lives in a busy pond full of bugs, lily pads and friends. Solve each problem to help Freddie out!
Check the work
Is each answer right? If it's wrong, write the right answer.
48
+ 49
87
Toby's answer isrightwrongRight answer:
56
− 19
43
Pip's answer isrightwrongRight answer:
34
+ 49
83
Fern's answer isrightwrongRight answer:
35
− 27
8
Toby's answer isrightwrongRight answer:
14
+ 79
83
Pip's answer isrightwrongRight answer:
50
− 36
26
Fern's answer isrightwrongRight answer:
Why does it work?
Circle the best reason.
To add 38 + 25, you can add 2 to 38 and take 2 from 25. That works because…the total stays the sameyou get a new total
To take 9 from 54, you can take 10 and add 1 back. That works because…9 is 1 less than 109 is 1 more than 10
In 46 + 37, the ones make 13, so you write 3 and carry 1. The 1 is really…one tenone one
Sheet
Freddie's Frog Pond
Grade 2 · Checking strategies · 2.NBT.B.9
Answer key
Freddie the frog lives in a busy pond full of bugs, lily pads and friends. Solve each problem to help Freddie out!
Check the work
Is each answer right? If it's wrong, write the right answer.
48
+ 49
87
Toby's answer isrightwrongRight answer: 97
56
− 19
43
Pip's answer isrightwrongRight answer: 37
34
+ 49
83
Fern's answer isrightwrongRight answer: 83
35
− 27
8
Toby's answer isrightwrongRight answer: 8
14
+ 79
83
Pip's answer isrightwrongRight answer: 93
50
− 36
26
Fern's answer isrightwrongRight answer: 14
Why does it work?
Circle the best reason.
To add 38 + 25, you can add 2 to 38 and take 2 from 25. That works because…the total stays the sameyou get a new total
To take 9 from 54, you can take 10 and add 1 back. That works because…9 is 1 less than 109 is 1 more than 10
In 46 + 37, the ones make 13, so you write 3 and carry 1. The 1 is really…one tenone one
Answer key
New problems every time, three levels (gentle, core and stretch), themes to pick from, a second page if you want one. Each sheet comes with an answer key and a one-page guide for grown-ups.
