Collecting like terms: the dropped minus sign
Your child groups the y terms correctly, then writes 5 − 8 as 3 and the minus sign disappears. The expression looks almost right, so it is easy to miss when checking. If the question then asks for a value of y, that answer is wrong as well.
What it looks like
A box had (7y + 5) sweets. Farah took out 3y sweets and then 8 more sweets. (a) How many sweets were left in the box? Give your answer in terms of y. (b) Find the number of sweets left when y = 4.
- (a) 7y + 5 − 3y − 8
- = 7y − 3y + 5 − 8
- = 4y − 3
- (b) 4 × 4 − 3 = 16 − 3 = 13
Why (a) 4y + 3 (b) 19 is wrong: Your child worked out 5 − 8 as 3 instead of −3, so the expression became 4y + 3 and 4 × 4 + 3 = 19.
Why children do it
- Children are used to subtracting the smaller number from the bigger one, so 5 − 8 becomes 8 − 5.
- Each sign belongs to the term after it, but children treat the signs as separate from the numbers.
- Letters and numbers sit side by side, and moving terms around without their signs is easy to do.
How to fix it at home
- Ask your child to circle each term together with the sign in front of it before collecting.
- Have your child rewrite the expression with the letter terms first, carrying the signs across.
- Check by substitution: put y = 4 into the original story (33 sweets, take away 12, take away 8) and compare.
Questions parents ask
P6 children have not learnt negative numbers. How do they handle 5 − 8?
They do not need negative numbers. Taking away 8 after adding 5 is the same as taking away 3, so the expression is 4y − 3.
Is algebra a big part of the PSLE?
It is a small topic, but algebra questions are usually short and straightforward, so these are marks your child should not drop.
Where it shows up
Other careless mistakes
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