A hall is set up with identical tables. Each table has n chairs around it. There are 4 tables, and 5 more chairs against the wall, which is (4n + 5) chairs. Find how many chairs there are when n = 6.
There are 29 chairs in the hall.
Ans: 29
Example 1
Find the value of 5n − 4 when n = 7.
5n − 4 = 31 when n = 7
Ans: 31
How to solve it
Read what the question says the letter stands for. Here n is 6.
Write the whole expression again with that number in place of the letter: 4 n plus 5 becomes 4 times 6, plus 5.
Change every n at the same time. Do not swap the first n and leave the others.
Leave everything else untouched. The 5 comes across as it is.
Only then work it out: 4 times 6 is 24, and 24 plus 5 is 29 chairs.
The question told us what the letter stands for, so we wrote that number in its place, kept the whole expression, and only then worked it out. Same three steps, every time.
Practice questions with worked solutions
Question 1 is free to try. The other 7 open in the free trial, each with its full working.
How do you answer algebra substitution questions in P6?
Substitution is the syllabus's own word for putting a known value in place of the letter and working out what the expression is worth.
What your child learns here
Substitution is the syllabus's own word for putting a known value in place of the letter and working out what the expression is worth. On this Primary 6 page the expressions include letter terms, plain numbers and the stacked fraction form.
Read what value the question gives the letter, and say it the house way: we now substitute n to be 5.
Write the whole expression again with the number in place of every copy of the letter, before any arithmetic.
Remember that 3n means 3 × n, so with n = 4 it is worth 12, not 34. Some schools write it as 3(4), which means the same.
Work out the value using the order of operations: multiply and divide before adding and subtracting.
Read a fraction bar as an instruction: in 5n over 4, the whole of 5n is divided by 4.
Does the answer make sense?
Your child's final answer is one plain number with no letter left in it. As a size check, 3n with n as 4 is worth 12, so an answer that looks like the digits written side by side, such as 34, means the hidden times sign was skipped.
How to practise these questions
Basic questions train replacing the letter with its number and then finishing the arithmetic in the correct order. Intermediate questions add collecting like terms before substituting, and expressions set out as fractions. Challenge questions involve two letters, one described using the other, or several expressions from which your child must choose. If putting the number back in goes off track, read the written step-by-step solution.
Questions parents ask
What does substitute mean in algebra? It means putting a given number in place of the letter and working out the value. The Primary 6 syllabus calls this evaluating simple linear expressions by substitution. In our lessons we say, for example, 'we now substitute n to be 5'.
My child's school writes 3(4) instead of 3 × 4. Is that the same? Yes. Some schools write a number in brackets straight after another number to mean multiply, so 3(4) is 3 × 4 = 12. Our pages write the times sign, and both forms are correct.
Which part of the expression does my child work out first? The usual order of operations applies: multiplication and division before addition and subtraction. A term like 3n is a multiplication, so it is worked out before anything is added to it or taken from it.
