Ever wondered how to find a price nobody told you?
Tunde bought 3 exercise books and a ₦200 biro, and paid ₦1,100 altogether. He forgot the price of one book. With one letter and one equation, you can find it in three lines.
Revision notes
Turn a story about prices, ages, perimeters or numbers into an equation, solve it, and check the answer in the story.
Tunde bought 3 exercise books and a ₦200 biro, and paid ₦1,100 altogether. He forgot the price of one book. With one letter and one equation, you can find it in three lines.
Read the story and decide what you do not know. Give it a letter: let the price of one book be ₦x.
Write every other quantity using x. 3 books cost 3x naira; with the biro the total is 3x + 200.
The story tells us that total is ₦1,100, so the equation is 3x + 200 = 1100.
🔑 Key idea: Let x be the unknown, then write the story in symbols.
An equation is a balance. Whatever you do to one side, do to the other.
Subtract 200 from both sides: 3x = 900. Divide both sides by 3: x = 300, so one book costs ₦300.
Check in the story: 3 × ₦300 + ₦200 = ₦1,100 ✓.
🔑 Key idea: Undo + and − first, then undo × by dividing. Always check in the story.
Aisha buys 3 plates of jollof rice at the same price and one bottle of water for ₦250. She pays ₦1,750 altogether. Form an equation and find the price of one plate of jollof rice.
Step 1: Choose the unknown
Let the price of one plate of jollof rice be ₦x.
Step 2: Form the equation
3x + 250 = 1,750
Step 3: Subtract 250 on both sides
3x = 1,500
Step 4: Divide both sides by 3
x = 1,500 ÷ 3 = 500
Step 5: Check in the story
3 × 500 + 250 = 1,750 ✓
Why? Always put your answer back into the WORDS of the problem, not just the equation.
Answer: ₦500
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.