Revision notes
Use a letter like x to stand for a number you don't know yet — then write it and work it out.
Mama Chidi sells oranges in closed bags. Every bag holds the same number, but nobody has opened one yet. How can she write down how many oranges are in 3 bags? Algebra gives her a smart shortcut: a letter.
We don't know how many oranges are in one bag, so we give that number a name: x.
A letter that stands for an unknown number is called a variable or an unknown.
x is not a new kind of thing — it is just a number we haven't found yet. Any letter will do: a, n, p or y.
🔑 Key idea: A letter stands for a number we don't know yet.
3 bags hold x + x + x oranges. We write this as 3x, which means 3 × x. The number goes in front and the × sign is left out.
If Mama Chidi also has 2 loose oranges, she has 3x + 2 oranges altogether. This is called an expression.
Key words help: "4 more than x" is x + 4, "4 less than x" is x − 4, and "x shared among 4" is x ÷ 4.
🔑 Key idea: 3x means 3 × x. "More than" means add; "less than" means subtract.
Once we find out what x is, we can work the expression out. If one bag holds 10 oranges, then x = 10.
So 3x + 2 = 3 × 10 + 2 = 30 + 2 = 32 oranges.
Careful: 3x with x = 10 is 3 × 10 = 30, not 310. Always multiply before you add.
🔑 Key idea: Put the number in place of the letter, then multiply before you add.
Every basket holds the same number of mangoes, but we don't know how many — so we call it x. Chidi has 6 full baskets and 4 loose mangoes, so Chidi has 6x + 4 mangoes altogether. Chidi opens one basket and counts 15 mangoes, so x = 15. How many mangoes does Chidi have altogether?
Step 1: Put the number in place of the letter
6x + 4 = 6 × 15 + 4
Why? x stands for the number in one basket, which is 15.
Step 2: Multiply first, then add
= 90 + 4 = 94
Answer: 94
Ready to check you've got it? Scroll up and work through the lesson: the interactive visuals, practice questions and mastery check are all there.