Kai adds 604 + 356 in columns and gets 950. What mistake did Kai make? A They added the ones digits wrongly B They took away instead of adding C They forgot to exchange 10 ones for 1 ten D They forgot to exchange 10 tens for 1 hundred

C · They forgot to exchange 10 ones for 1 ten

You work out 478 + 322 in columns. In which columns do you exchange 10 for 1 in the next column? A Only the ones column B The ones and the tens columns C Neither column D Only the tens column

B · The ones and the tens columns

Maths Year 3 Addition & subtraction

The objective: Add and subtract numbers up to 3 digits using formal columnar methods.

The explanation, example and questions are the ones Remelia uses in the app; the questions are not taken from any test paper.

Column addition and subtraction line the numbers up so that every column holds the same place value, then you work right to left.

Write the numbers one above the other, ones under ones.
Start at the right-hand column.
Adding: if a column makes 10 or more, write the ones digit and exchange the ten for 1 in the next column.
Subtracting: if the top digit is too small, exchange one from the column to the left.

342 − 56: ones 2 − 6 needs an exchange, so 12 − 6 = 6; tens are now 3 − 5, exchange again, 13 − 5 = 8; hundreds 2. Answer 286.

Not lining up the columns when the numbers are different lengths. In 342 + 56, the 5 goes under the 4, not under the 3.

When a wrong answer to a question like these matches one of these mistakes, the app says what the child seems to have done before they try again.

Question 1, the answer 278: It looks as if you took the smaller digit from the bigger one, even where the bigger one was at the bottom. Where the top digit is smaller, exchange first: take 1 from the next column to the left, which makes 10 more in this one, then subtract.
Question 2, the answer 1,321: It looks as if a column came to 10 or more and nothing was exchanged. Write its ones digit, and exchange the ten for 1 in the next column to the left.