Algebra: 3a is not 35
Algebra at P6 is small, but every mark in it is lost the same few ways. A letter is treated as a digit (3a with a = 5 becomes 35), words are turned into the wrong expression, or a simple equation is solved one step short.
What it looks like
Find the value of 3a + 7 when a = 5.
- 3a means 3 × a = 3 × 5 = 15
- 15 + 7 = 22
Why 42 is wrong: 3a was read as the two-digit number 35, so 35 + 7 = 42.
Why children do it
- Writing 3a with no sign between the 3 and the a hides the multiplication.
- Phrases like "5 more than twice x" give the numbers in a different order from the expression (2x + 5).
- Solving an equation is new, and the last step (dividing to find one x) gets skipped.
How to fix it at home
- Have your child rewrite 3a as 3 × a before substituting, every time, until it is automatic.
- Turn words into pictures first: a bar for x, then what is added or doubled. Write the expression from the picture.
- After solving, substitute the answer back into the original equation and check both sides match.
Questions parents ask
How much algebra is in the PSLE?
Expressions with one letter, simplifying, substituting and simple equations with whole-number answers. A few marks, but reliably there.
Should my child use algebra for problem sums?
Only where it is natural. Bar models are still the main method; algebra questions in the PSLE are usually direct.
How does Knowbotic catch it?
The 35-style answer is a known trap on substitution questions, so a wrong answer is named as this habit and practised until fixed.
Where it shows up
Other careless mistakes
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