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Class 7 Finding the Unknown: cheat sheet
CBSE Mathematics. An equation is a balance. Do the same thing to both sides to find the unknown, and turn word problems into equations.
Key ideas
- An equation says two expressions are equal; solving it means finding the value that makes it true.
- Keep the balance: add, subtract, multiply or divide both sides by the same number.
- Undo the operations in reverse order: first the adding or subtracting, then the multiplying or dividing.
- Check the answer by putting it back into the original equation.
- For a word problem, choose a letter for the unknown, write the equation from the facts, solve it, and answer in words.
Remember
| Example | 4b + 10 = 66 → 4b = 56 → b = 14 |
| Perimeter of a rectangle | 2(l + b) |
| Consecutive numbers | n, n + 1, n + 2 |
Watch out
- Doing an operation to one side only.
- Dividing before subtracting, as in 4b + 10 = 66 → b + 10 = 16.5.
- Stopping at the value of the letter without answering the question (for example, giving the breadth when the length was asked).
Worked example
The length of a rectangle is 5 cm more than its breadth. Its perimeter is 66 cm. Find the length and breadth.
Working: 2(b + 5 + b) = 66 → 4b + 10 = 66 → 4b = 56 → b = 14
Answer: breadth = 14 cm, length = 19 cm
Practise Finding the Unknown
Revise with this page, then practise: every Finding the Unknown sheet in the app is new, with an answer key and working.
Coming soon for Android and iPhone.