Substitute one supplied value and calculate the expression.
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Practise it now
Get ONE mystery bag alone on a pan - keeping the scale level - and you have found x.
It covers most of this skill rather than all of it.
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A worked example
One question of the kind this skill is tested with, worked all the way through: the reasoning, and what the wrong answers are there to catch.
Putting a value in place of a letter
Point A is marked on the number line, and n is the value at A. What is 4n - 8?
Each small step on this line is worth 1, and every labelled tick is ten steps on.
28
12
37
1
Answer: 12
How you get there
Reading the line and substituting gives 4n - 8 = 12: substitute n = 5, so 4n means , and taking 8 away gives 12. A letter written next to a number always means multiply, so the multiplication happens before the addition or subtraction unless a bracket says otherwise.
Why the other answers tempt
A number written next to a letter means multiply by it, not add it.
If they are stuck, say this
Read A off the line first, then put the value in place of the letter before you do any arithmetic.
Get ONE mystery bag alone on a pan - keeping the scale level - and you have found x.
A balanced scale is an equals sign made of metal. 2x + means the two pans genuinely weigh the same. Remove 3 from both pans and it is still level: 2x = 8. Halve both pans and it is still level: x = 4. Every step of algebra is just something you did to both sides at once.
What to say if they get stuck
Take a weight off one pan and watch what happens. The beam is telling you something.
Whatever you take from one side, take the SAME from the other. Then it stays level.
Once only bags are left on one side, split BOTH sides into that many equal groups.
Work out which two gears turn these numbers into those numbers.
This engine’s own main skill is Function Machines & Inverse. It practises evaluate an algebraic expression alongside it.
A function machine does the same thing to every number that enters it. You found the rule backwards - from the outputs - which is inverse thinking. And the order of the gears is part of the rule: ×3 then +1 turns 2 into 7, while +1 then ×3 turns the same 2 into 9. Same gears, different machine.
Step 1. Deduce the gears. Drag gears into the machine so every ticket comes out right.
Step 2. Run it. New number going in. Predict the output BEFORE you pull the lever.
What to say if they get stuck
Look at 2 → 7 and 4 → 13. When the input goes up by 2, the output jumps by 6. What multiplies by 3?
Try ×3 first: 2 becomes 6, but the ticket says 7. What tiny gear finishes the job?
The order of the gears changes the answer. ×3 then +1 gives 7; +1 then ×3 gives 9.
How evaluate an algebraic expression differs from the skills beside it
The same exercise practises several skills, and telling them apart is often the difficulty rather than the exercise itself. On this page the skill is: substitute one supplied value and calculate the expression, and it stops where excludes solving an equation.
Apply one arithmetic mapping to a letter or number code. One invariant rule only.
Questions parents ask about evaluate an algebraic expression
Answered from what the skill map records about this skill, not from a template. Every answer here is on the page, and the same questions are the page's FAQ structured data.
Substitute one supplied value and calculate the expression. It is one of 55 named maths skills in the 11+, in the algebra strand, and this page carries 2 interactive exercises that practise it.
A number written next to a letter means multiply by it, not add it. The wrong answers on this skill are built out of that, not out of random numbers, so it is worth naming out loud before your child meets it under time pressure.
Point at the next move rather than explaining the whole rule. Read A off the line first, then put the value in place of the letter before you do any arithmetic.
The skill map draws the line here: Excludes solving an equation. A question past that line is testing a different named maths skill, which is worth knowing before you spend a week practising the wrong one.
The algebra strand holds 2 skills that are tested together, and the page next to evaluate an algebraic expression is use a term rule. Each has its own exercise and its own link, and none of them needs an account.
Use Balance Scale and Function Machine, embedded on this page. They are the exercises the skill map attaches to evaluate an algebraic expression, they run in the browser with no account to make, nothing is scored, and nothing a child does here is recorded or sent anywhere. Each also has its own link, so you can send one to a child without sending them anywhere else first.
Knowing which maths topic to practise is most of the work
A free diagnostic covers arithmetic, fractions, geometry, measure and data handling, and comes back with the topics to spend the next few weeks on.
